Improve testing and fix bugs for LocalFrameUKMAggregator

Add testing for the ratio metrics and fix bugs that were detected.

Rename the ratio metrics to be specific to MainFrame to prepare for future ratios.

R=pdr@chromium.org
BUG=892861

Change-Id: I01ad5d059d423366e118335ff9288e7eb2cdb339
Reviewed-on: https://chromium-review.googlesource.com/c/1320692
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606584}
3 files changed