Added merge session request throttle for XHR requests.

BUG=308318,309131,308681
TEST=manual, added MergeSessionTest.MergeSessionThrottle

Review URL: https://codereview.chromium.org/118733002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242135 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242135