Add duplicate libcxx source repo paths.

Context: go/unnesting-buildtools-lib
We are moving these source files buildtools/third_party/lib*/trunk
-> third_party/libcxx/lib*/trunk.

rollout plan is:
1. This Cl
2. in chromium/src merge
https://chromium-review.googlesource.com/c/chromium/src/+/4666325
3. remove old buildtools/third_party/lib*/trunk paths in DEPS

work tracked here: http://shortn/_BqvAmLrzlr

Bug: chromium:1458042
Change-Id: Idde9b7650923a2f21380db4c73d58b76198928e5
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/110110
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

DEPS

diff --git a/DEPS b/DEPS
index e9eaf3b..9712aa3 100644
--- a/DEPS
+++ b/DEPS
@@ -254,6 +254,8 @@
     'dep_type': 'cipd',
   },
 
+  # TODO(chromium:1458042): Remove these paths, when chromium builds files
+  # have moved to third_party/libcxx/lib*/trunk paths.
   'buildtools/third_party/libc++/trunk':
     Var('chromium_git') +
         '/external/github.com/llvm/llvm-project/libcxx.git@' +
@@ -343,6 +345,21 @@
     Var('chromium_git') + '/chromium/src/third_party/jinja2.git@' +
         Var('jinja2_revision'),
 
+  'third_party/libcxx/libc++/trunk':
+    Var('chromium_git') +
+        '/external/github.com/llvm/llvm-project/libcxx.git@' +
+        Var('libcxx_revision'),
+
+  'third_party/libcxx/libc++abi/trunk':
+    Var('chromium_git') +
+        '/external/github.com/llvm/llvm-project/libcxxabi.git@' +
+        Var('libcxxabi_revision'),
+
+  'third_party/libcxx/libunwind/trunk':
+    Var('chromium_git') +
+        '/external/github.com/llvm/llvm-project/libunwind.git@' +
+        Var('libunwind_revision'),
+
   'third_party/libjpeg_turbo':
     Var('chromium_git') + '/chromium/deps/libjpeg_turbo.git@' +
         Var('jpeg_turbo_revision'),