Pdfium_unittests: Fix undefined reference to testing::Matcher

The pdfium_unittest target currently fails to build because of an
undefined reference to gtest's matchers.

Fix this by including gtest-matchers.{h,cpp} in gtest's source_set.

Change-Id: I4dffd1e924cc48b013504dcdfdf7b702c47b4780
Reviewed-on: https://pdfium-review.googlesource.com/c/50451
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/third_party/googletest/BUILD.gn b/third_party/googletest/BUILD.gn
index e2b4bd6..2bde37c 100644
--- a/third_party/googletest/BUILD.gn
+++ b/third_party/googletest/BUILD.gn
@@ -44,6 +44,7 @@
   testonly = true
   sources = [
     "src/googletest/include/gtest/gtest-death-test.h",
+    "src/googletest/include/gtest/gtest-matchers.h",
     "src/googletest/include/gtest/gtest-message.h",
     "src/googletest/include/gtest/gtest-param-test.h",
     "src/googletest/include/gtest/gtest-printers.h",
@@ -67,6 +68,7 @@
     "src/googletest/src/gtest-death-test.cc",
     "src/googletest/src/gtest-filepath.cc",
     "src/googletest/src/gtest-internal-inl.h",
+    "src/googletest/src/gtest-matchers.cc",
     "src/googletest/src/gtest-port.cc",
     "src/googletest/src/gtest-printers.cc",
     "src/googletest/src/gtest-test-part.cc",