Support Drag and Drop across displays.

This CL introduces DragDropTracker, which
- Makes a dummy window capture events while dragging an item, and
- Dispatches mouse events to the window at the pointer location in DragDropController::PreHandleMouseEvent.

BUG=136817


Review URL: https://chromiumcodereview.appspot.com/10855159

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@153401 0039d316-1c4b-4281-b951-d872f2087c98
14 files changed