<br> nodes should not be allowed within SVG text

Add new method isRenderableTextNode to SVGRenderSupport to filter out any
RenderObjects that return true for isText(), but is not recognized by the
SVG text rendering codepath.

BUG=339993, 337607

Review URL: https://codereview.chromium.org/145493012

git-svn-id: svn://svn.chromium.org/blink/trunk@166505 bbb929c8-8fbe-4397-9dbb-9b2b20218538