Changed fake callbacks to post tasks asynchronously.

This catches bugs related to parameter lifetime that are not caught when
the Run() method is called directly.  See crbug.com/315624 for an
example.

BUG=chromium:316249
TEST=ran unit tests with and without fix for crbug.com/315624
R=nkostylev@chromium.org

Review URL: https://codereview.chromium.org/93383002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238512 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238512