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This CL enables using libfuzzer on linux coverage tot builder, however,
due to crbug.com/790747, fuzzer targets won't compile on Mac yet.

Change-Id: I9bde883c382f476811883f3b5b9c05424b23c1e1
Reviewed-on: https://chromium-review.googlesource.com/1078070Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563100}