Enforce resizing a single window at a time part 1.

This CL prevents starting a new drag if one is already in progress. This is the first part in making sure that a window resizer created by ToplevelWindowEventHandler and one created by MultiWindowResizeController do not coexist.

BUG=324038
TEST=ToplevelWindowEventHandlerTest.RunMoveLoopFailsDuringInProgressDrag

Review URL: https://codereview.chromium.org/147953002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247293 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247293