Disable PageThrottle and XHRThrottle in MergeSessionTest.

These tests are failing. See bug.

TBR=zelidrag@chromium.org
BUG=339671

Review URL: https://codereview.chromium.org/144523003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248099 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248099