app_list: Defer setting drop target on hovering page button.
- Do not set drop target to new page on hovering page button;
- Defer that until drag is committed (EndDrag is called) or page switching
is finished;
BUG=159420
TEST=Verify dragging over a page button does not moving icons and drop target
until user drops the icon or page is flipped.
R=sky@chromium.org
Review URL: https://chromiumcodereview.appspot.com/
11365135
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@166575 0039d316-1c4b-4281-b951-d872f2087c98