Removes MessageLoop::Type checks in favor of IsCurrent on MessageLoops.

This is part of removing the MessageLoop::Type enum.

BUG=none
TEST=none
R=darin@chromium.org

Review URL: https://codereview.chromium.org/141683005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245882 0039d316-1c4b-4281-b951-d872f2087c98