Set nestable tasks allowed to true before running windows move loop.
This is required for when the move loop is started using touch because we
handle touch events asynchronously during the move loop.

BUG=279317
R=sky@chromium.org

Review URL: https://codereview.chromium.org/24286006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224718 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224718