Be consistent about writing modes in boundingBoxForCompositingOverlapTest().

Always use "physical" rectangles (i.e. flip for writing-mode). This is what
boundingBoxForCompositing() already uses.

BUG=423372

Review URL: https://codereview.chromium.org/653973002

git-svn-id: svn://svn.chromium.org/blink/trunk@183694 bbb929c8-8fbe-4397-9dbb-9b2b20218538