Share more code between RenderSVGText and RenderSVGForeignObject

It turns out thse two classes share a lot of code, so move it to the common
base class RenderSVGBlock

Review URL: https://codereview.chromium.org/136723007

git-svn-id: svn://svn.chromium.org/blink/trunk@165097 bbb929c8-8fbe-4397-9dbb-9b2b20218538