Implement canHaveWhitespaceChildren on RenderSVG{Root,ModelObject}

Make said method return false to signal that we don't care about
whitespace-only Nodes in these contexts.
This avoids creating large number of text RenderObjects
(RenderSVGInlineTexts) and also avoids hitting the linear search
at the bottom of Text::textRendererIsNeeded.

R=pdr,schenney,fmalita
BUG=339470,337607

Review URL: https://codereview.chromium.org/136963005

git-svn-id: svn://svn.chromium.org/blink/trunk@166238 bbb929c8-8fbe-4397-9dbb-9b2b20218538