Added CHECK for tasks passed to PostTask being null.

I hope this will make the crashes associated with posting null tasks fail fast so they can be more easily diagnosed.

TEST=try
BUG=none
TBR=darin@chromium.org
Review URL: http://codereview.chromium.org/6982004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@84869 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed