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Previously, we flattened all the iterations and reported one number. Now we
report the geo mean of each iteration so that the dashboard can display the
stddev.

BUG=405719

Review URL: https://codereview.chromium.org/493153002

Cr-Commit-Position: refs/heads/master@{#291248}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291248 0039d316-1c4b-4281-b951-d872f2087c98