Adds SVGImageElement to CanvasImageSource

Relevant spec: https://html.spec.whatwg.org/multipage/scripting.html#image-sources-for-2d-rendering-contexts:htmlorsvgimageelement

SVGImageElement should be part of CanvasImageSource union typedef, i.e., be allowed as an source for drawImage(). This CL does that, while creating a common interface for both HTMLImageElement and SVGImageElement.

BUG=695662

Review-Url: https://codereview.chromium.org/2723093004
Cr-Commit-Position: refs/heads/master@{#459289}
19 files changed