Cache feedback reports to disk in case of send failure.

R=zork@chromium.org
BUG=249853

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=246992

Review URL: https://codereview.chromium.org/141433011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247772 0039d316-1c4b-4281-b951-d872f2087c98
telemetry: Compute discrepancy per timeline range and return max.

This patch changes all metrics in RenderingStats to be lists of lists
(one per timeline range). Right now, we only need this feature for frame_timestamps,
but it is cleaner to handle all metrics the same way and it preserves more
information.
The discrepancy calculation was modified to handle list-of-list inputs
correctly.

R=nduca@chromium.org,dominikg@chromium.org
BUG=338796

Review URL: https://codereview.chromium.org/149093003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247773 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247773