Compute and use BiDi information in SVGTextMetricsBuilder

This extends upon https://codereview.chromium.org/104813005, by computing
(and plumbing) directionality information for SVG text elements.
The method used is similar to that used by
RenderText::computePreferredLogicalWidths although simplified somewhat.

BUG=366939

Review URL: https://codereview.chromium.org/253823002

git-svn-id: svn://svn.chromium.org/blink/trunk@172799 bbb929c8-8fbe-4397-9dbb-9b2b20218538