[Coverage] Enable experimental limited code coverage

This CL sets -limited-coverage-experimental=true to allow building
large unit test target on Mac.

Bug: 796290
Change-Id: Ief6277ae2ea3d4e1372c4fd796e97c2a1ae0bfba
Reviewed-on: https://chromium-review.googlesource.com/891799
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Abhishek Arya <inferno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532700}

build/config/coverage/BUILD.gn

@@ -10,6 +10,13 @@ config("default_coverage") {
       "-fprofile-instr-generate",
       "-fcoverage-mapping",
       "-fno-use-cxa-atexit",
+
+      # Following experimental flags removes unused header functions from the
+      # coverage mapping data embedded in the test binaries, and the reduction
+      # of binary size enables building Chrome's large unit test targets on
+      # MacOS. Please refer to crbug.com/796290 for more details.
+      "-mllvm",
+      "-limited-coverage-experimental=true",
     ]
     ldflags = [ "-fprofile-instr-generate" ]
   }