Make SVGTextPositioningElement::elementFromRenderer take a reference

This avoids doing a null check in this method, adjust
all call sites.

Review URL: https://codereview.chromium.org/715023002

git-svn-id: svn://svn.chromium.org/blink/trunk@185160 bbb929c8-8fbe-4397-9dbb-9b2b20218538