Let telemetry support X events

An X event combines a pair of B/E events into one event.

BUG=311381

Review URL: https://codereview.chromium.org/42563003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231139 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231139