[GCM] Fix connection factory unit tests timing

The BackoffEntry was using a real clock for timing, but this can result
in negative delays, particularly with valgrind slowness. This patch switches to a
manually controlled test clock for timing tests.

BUG=337560

Review URL: https://codereview.chromium.org/135923004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247322 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247322