Re-landing https://codereview.chromium.org/118733002:

Added merge session request throttle for XHR requests.

This is the second attempt which should also have fix for
regression that the original CL from r242135 have caused
(see http://crbug.com/333046 for details).

BUG=308318,309131,308681,333046
TEST=manual, added MergeSessionTest.MergeSessionThrottle
R=japhet@chromium.org
TBR=sky, xiyuan

Review URL: https://codereview.chromium.org/146953003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247454 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247454