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This is due to 2 reasons:
+ Page specific logic should belong to page, not page test.
+ Simplify the repaint measurement control code, which make
it easier to port repaint to TimelineBasedMeasurement.

Trybot link for repaint.gpu_rasterization.key_mobile_sites_repaint:
https://codereview.chromium.org/1098543002

BUG=444703, 455391

Review URL: https://codereview.chromium.org/1089223003

Cr-Commit-Position: refs/heads/master@{#325558}