Additional XHR merge session throttle tests.
Fixed event wait flakyness.

BUG=none
TEST=added MergeSessionTest.XHRThrottle, MergeSessionTest.NoXHRThrottle
TBR=asargent

Review URL: https://codereview.chromium.org/120403002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242859 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242859