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This CL hooks the Clang code coverage wrapper into the build
toolchains, and specifically, a new build flag:
coverage_instrumentation_input_file is added to allow turning on/off
the wrapper and pass the list-files-to-instrument around.

Bug: 898695
Change-Id: I405ccbfc1796afa44534794d711f2953fac78f6d
Reviewed-on: https://chromium-review.googlesource.com/c/1309999
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604858}