Re-enabled fixed MergeSessionTest(s).

BUG=339671
TEST=MergeSessionTest.*

Review URL: https://codereview.chromium.org/131153008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248249 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248249