Make <iframe> containing SVG behave as every other <iframe>

When an SVG is contained inside an <iframe>, its dimensions should not
be part of size calculations of the <iframe>. Neither should the SVG's
own width and height be ignored when sizing the <svg> inside the
<iframe>

The specification waters are murky here but this aligns Blink with
Firefox and IE.

BUG=308992,364051

Review URL: https://codereview.chromium.org/246933007

git-svn-id: svn://svn.chromium.org/blink/trunk@172467 bbb929c8-8fbe-4397-9dbb-9b2b20218538