webkit-app-region: Make sure the bounds is properly transformed.

The draggable-regions reported by a Document can be incorrect if the element that
has -webkit-app-region set also has transforms (e.g. rotate, scale). Make sure the
entire bounds of the element is transformed instead of just the origin to fix this.

BUG=none
R=hartmanng@chromium.org, vollick@chromium.org

Review URL: https://codereview.chromium.org/25086003

git-svn-id: svn://svn.chromium.org/blink/trunk@158510 bbb929c8-8fbe-4397-9dbb-9b2b20218538