base: Remove should_leak_tasks_ from MessageLoop

The flag is no longer referenced.

BUG=61131
TEST=base_unittests

Review URL: http://codereview.chromium.org/9903003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@129586 0039d316-1c4b-4281-b951-d872f2087c98