[CSS Shapes] SVG Image valued shape fails if root element's size is relative

When creating a shape based on an Image, compute it's dimensions using RenderBoxModelObject::calculateImageIntrinsicDimensions(), then set the ImageResource's containerSize, and then use imageForRenderer() to get the image to be drawn. The first step resolves SVG Image relative sizes, the second step creates and caches an SVGImageForContainer with the correct containerSize, and the last step retrieves the SVGImageForContainer.

BUG=344579

Review URL: https://codereview.chromium.org/183803035

git-svn-id: svn://svn.chromium.org/blink/trunk@168733 bbb929c8-8fbe-4397-9dbb-9b2b20218538