wm: Modify DragWindowController to account for a transform

The present CL modifies DragWindowController to account for a transform
on the dragged window. This change solves Issue 965665 because in tablet
mode, the visual of dragging a window from the top is achieved with a
transform (except for tab dragging).

Bug: 965665
Change-Id: I2d75f203646aae1630bf8575bae2bdf4e80afcbf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1623406
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662313}