aura: Remove an unnecessary parameter from DispatcherClient::RunWithDispatcher.

The |nestable_tasks_allowed| parameter was always being set to true from the
callsites. So get rid of the variable, and use ScopedNestableTaskAllower in
the RunWithDispatcher implementations.

BUG=none
R=sky@chromium.org

Review URL: https://codereview.chromium.org/162633002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251072 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251072