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Origianlly, the code coverage script uses "llvm-cov export" to get
the code coverage, but the problem was that it took too long to run
and contains a lot of unrelated information, and for example, it took
more than 5 minutes to generate code coverage data for
ios_chrome_unittests.

This CL speeds up the report generation by changing to use
"llvm-cov report", it's more hacky, but it's significantly faster. 
This change reduced the time to generate code coverage report for 
ios_chrome_unittests from more than 5 minutes to less than 1 minute.

Bug: 763956
Change-Id: Ib62a29fef67ea6973e063893af2164b34a25841c
Reviewed-on: https://chromium-review.googlesource.com/661677
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Mike Baxley <baxley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501324}