DevTools: Add timestamp to the trace event callback.

That is needed for upcoming GPU events arriving in batches from
GPU process where they had already been set up with timestamps.
So setting timestamps on Blink side would be too late.

Blink counterpart is at https://codereview.chromium.org/40523003/

Review URL: https://codereview.chromium.org/40573002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231271 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231271