Change GraphicsContext::createCompatibleBuffer to return something non-null

The former implementation asked the current SkCanvas, in all cases now a
SkPictureRecord, for a new SkSurface. The SkPictureRecord always returned null for
the SkSurface. Now we just return a new bitmap surface regardless.

There are two pieces of code that depend on this: cross-fade and SVG
buffered-rendering (which apparently is going away, see crbug.com/395766).
For some reason the cross fade code worked anyway, maybe due to some
fallback path, but the SVG caching didn't.

Note we don't want any of this rasterization during recording. But for
now it stays.

R=fmalita@chromium.org,reed@google.com
BUG=394765,394768

Review URL: https://codereview.chromium.org/399213002

git-svn-id: svn://svn.chromium.org/blink/trunk@178448 bbb929c8-8fbe-4397-9dbb-9b2b20218538