This is a re-land of https://codereview.chromium.org/2901383002, which was
reverted due to a timing dependency in ResourcePrefetchPredictorBrowserTest.
SubresourceFcpOrder that has since been fixed (crbug.com/728176).

Original change description:

With the recent change to merge cross-frame paint timings, we sometimes encounter
cases where these timings arrive out of order.

For example, we may receive a first paint notification for frame A first,
then a first paint notification for frame B, but frame B's first paint
time may have happened before frame A's first paint time.

In the current implementation, we dispatch a first paint notification to observers
as soon as we receive a first paint notification from any frame in the page.

This means we don't end up recording the true page-level first paint time on
all pages.

This change adds a 1s buffering period before we dispatch first paint notifications
to observers. During this buffering period, we accumulate all first paint updates
across all frames, and retain the minimum first paint time. We then dispatch the min
observed first paint time across this window.

This change should significantly reduce the frequency with which we log out of order
paint timings. We have added new UMA to keep track of how often this continues to
happen, even with the 1s buffering.

BUG=728179
TBR=isherman,mattcary

Review-Url: https://codereview.chromium.org/2917693002
Cr-Commit-Position: refs/heads/master@{#475955}