Mac: Fix crash when dragging tabs

When dragging the only tab of a window, and there are other windows that are
potential targets, it was possible in some cases for the window to deallocate
while we still hold a weak reference to it.

BUG=355471

Review URL: https://codereview.chromium.org/199663004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@259773 0039d316-1c4b-4281-b951-d872f2087c98