Keep text position locally in SVGTextMetricsBuilder::measureTextRenderer

Passing m_textPosition as an argument to the various helper methods
allows it to be removed from SVGTextMetricsBuilder and stored on the
stack of measureTextRenderer.

Review URL: https://codereview.chromium.org/182733010

git-svn-id: svn://svn.chromium.org/blink/trunk@168618 bbb929c8-8fbe-4397-9dbb-9b2b20218538