HTMLImageElement.natural{Width,Height} should return intrinsic dimensions

HTML says:

 "The IDL attributes naturalWidth and naturalHeight must return the
  intrinsic width and height of the image, in CSS pixels, if the image is
  available, or else 0."

Change HTMLImageElement::natural{Width,Height} to request the intrinsic
size from ImageResource::imageSizeForRenderer by passing IntrinsicSize as
the third parameter. This makes the attributes return more appropriate
values for SVG-images in <img>.

BUG=396955

Review URL: https://codereview.chromium.org/422503002

git-svn-id: svn://svn.chromium.org/blink/trunk@179030 bbb929c8-8fbe-4397-9dbb-9b2b20218538