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After setting emma_coverage as true, and then run JUnit testcases, code
coverage information will be stored in coverage.ec. In order to read
coverage.ec file, you will need to find the corresponding <name>.em file
in out-gn/Debug and then merge the .em and .ec file into a txt or html.

BUG=604900

Review URL: https://codereview.chromium.org/1913593002

Cr-Commit-Position: refs/heads/master@{#389902}