Return transform from transformationForTextBox instead of using out-param

Also simplify SVGTextChunkBuilder::transformationForTextBox while in
the neighborhood.

Review URL: https://codereview.chromium.org/370803002

git-svn-id: svn://svn.chromium.org/blink/trunk@177774 bbb929c8-8fbe-4397-9dbb-9b2b20218538