Keep the oldest LatencyInfo when coalescing input events

Input events can be coalesced at browser side, e.g. GestureScrollUpdate,
before being sent to renderer. Currently when events are coalesced, LatencyInfo
are merged. We want to move to per-event LatencyInfo tracking and get rid
of LatencyInfo merge. It seems reasonable to keep the oldest LatencyInfo, which
when used for input event latency analysis, will represent the longest latency.

BUG=246034
TEST=None

Review URL: https://codereview.chromium.org/86603003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240160 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240160