Telemetry: Unique timeline marker names.

Make sure that timeline markers inserted into the trace by Telemetry have unique
names. This is needed to find markers corresponding to a specific page action.

BUG=322067

Review URL: https://codereview.chromium.org/109433004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240281 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240281