Adding a way to remove an intent callback.

Adding a method that allow removal of callbacks registered for an intent
result.

BUG=None
TEST=None

Review URL: https://codereview.chromium.org/32543003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236952 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236952
Use base::PostTaskAndReplyWithResults() in more places.

Review URL: https://codereview.chromium.org/56833003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236953 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236953