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We want to measure the latency between a key event to the time of the
presentation of next UI frame. However, we do not save the time stamp in
ui::Accelerator.

This cl saves the time stamp information when ui::Accelerator is created
from KeyEvent.

Bug: 817116
Test: AcceleratorTest and AcceleratorStructTraitsTest
Change-Id: If4812a491e16371cf32adae8d7074232dc3c05d8
Reviewed-on: https://chromium-review.googlesource.com/956526Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542164}