Switch Chromium references to main branch.

Chromium switched to the main branch last week, so update references to
point to that branch.

Change-Id: I7c3699a7c0aa387fbed8dc5ba86487018ff672bc
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/79250
Commit-Queue: Lei Zhang <thestig@chromium.org>
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Auto-Submit: Lei Zhang <thestig@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
diff --git a/.vpython b/.vpython
index efe51d4..017ece5 100644
--- a/.vpython
+++ b/.vpython
@@ -9,7 +9,7 @@
 # CIPD (the "Chrome Infrastructure Package Deployer" service). Unlike `pip`,
 # this never requires the end-user machine to have a working python extension
 # compilation environment. All of these packages are built using:
-#   https://chromium.googlesource.com/infra/infra/+/master/infra/tools/dockerbuild/
+#   https://chromium.googlesource.com/infra/infra/+/main/infra/tools/dockerbuild/
 #
 # All python scripts in the repo share this same spec, to avoid dependency
 # fragmentation.
@@ -20,7 +20,7 @@
 #   vpython path/to/script.py some --arguments
 #
 # Read more about `vpython` and how to modify this file here:
-#   https://chromium.googlesource.com/infra/infra/+/master/doc/users/vpython.md
+#   https://chromium.googlesource.com/infra/infra/+/main/doc/users/vpython.md
 
 python_version: "2.7"
 
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 5f2a9f7..4c20a5d 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -1,6 +1,6 @@
 # CONTRIBUTING
 In general, we follow the
-[Chromium Contributing](https://chromium.googlesource.com/chromium/src/+/master/docs/contributing.md)
+[Chromium Contributing](https://chromium.googlesource.com/chromium/src/+/main/docs/contributing.md)
 guidelines in PDFium. The code review process, and the build tools are all very
 similar to Chromium. The PDFium
 [README](https://pdfium.googlesource.com/pdfium/+/refs/heads/master/README.md)
@@ -145,7 +145,7 @@
 discussing with a top-level owner). If the disagreement has moved up through
 all the OWNERS files in the PDFium repo, the escalation should then proceed to
 the Chromium
-[ENG_REVIEW_OWNERS](https://chromium.googlesource.com/chromium/src/+/refs/heads/master/ENG_REVIEW_OWNERS)
+[ENG_REVIEW_OWNERS](https://chromium.googlesource.com/chromium/src/+/refs/heads/main/ENG_REVIEW_OWNERS)
 as the final deciders.
 
 The
diff --git a/README.md b/README.md
index e8b1437..5c4472f 100644
--- a/README.md
+++ b/README.md
@@ -16,7 +16,7 @@
 
 #### Open source contributors
 Please refer to
-[Chromium's Visual Studio set up](https://chromium.googlesource.com/chromium/src/+/master/docs/windows_build_instructions.md#visual-studio)
+[Chromium's Visual Studio set up](https://chromium.googlesource.com/chromium/src/+/main/docs/windows_build_instructions.md#visual-studio)
 for requirements and instructions on build environment configuration.
 
 Run `set DEPOT_TOOLS_WIN_TOOLCHAIN=0`, or set that variable in your global
diff --git a/docs/getting-started.md b/docs/getting-started.md
index 6077d21..e5b261b 100644
--- a/docs/getting-started.md
+++ b/docs/getting-started.md
@@ -181,7 +181,7 @@
 
 
 
-[chrome-plugin]: https://chromium.googlesource.com/chromium/src/+/master/pdf/
+[chrome-plugin]: https://chromium.googlesource.com/chromium/src/+/main/pdf/
 [pdfium-public]: https://pdfium.googlesource.com/pdfium/+/master/public/
 [pdfium-v8]: /docs/v8-getting-started.md
 [pdfium-edit-guide]: /docs/pdfium-edit-guide.md
diff --git a/third_party/base/optional.h b/third_party/base/optional.h
index 65f3639..a380d4e 100644
--- a/third_party/base/optional.h
+++ b/third_party/base/optional.h
@@ -419,7 +419,7 @@
 // std::optional documentation:
 // http://en.cppreference.com/w/cpp/utility/optional
 // Chromium documentation:
-// https://chromium.googlesource.com/chromium/src/+/master/docs/optional.md
+// https://chromium.googlesource.com/chromium/src/+/main/docs/optional.md
 //
 // These are the differences between the specification and the implementation:
 // - Constructors do not use 'constexpr' as it is a C++14 extension.