Teach SVGImage::containerSize() to deduce intrinsic values

When an SVG has either intrinsic width or height but not both, and a
intrinsic ratio, the missing intrinsic width or height can be
calculated. Doing so enables us to return correct intrinsic width and
height for SVG images in more cases when the image has no renderer.

Also, this patch removes the unspecified use of the viewBox attribute
as the source of intrinsic size. A small number of tests depended on
this and has been updated. The reasoning to make this change is
because we don't use the viewBox for intrinsic size in any other
context than SVGImage. And we only use it until the renderer has a
containerSize() anyway.

http://www.whatwg.org/specs/web-apps/current-work/multipage/edits.html#dom-img-width
http://www.whatwg.org/specs/web-apps/current-work/multipage/edits.html#dom-img-height

BUG=376895

Review URL: https://codereview.chromium.org/308643002

git-svn-id: svn://svn.chromium.org/blink/trunk@175784 bbb929c8-8fbe-4397-9dbb-9b2b20218538