Handle CSSImageValue URLs in inline styles in templates correctly

When importing or adopting nodes across Documents, this patch checks for
baseURL mismatches between the two documents and re-resolves URLs for
CSSImageValues in those cases. Without this patch, the URLs are empty
when they enter the Document (since they tried to resolve against an
empty baseURL when they were created).

This requires storing the un-resolved URL inside the CSSImageValue alongside
the resolved URL, and some unfortunately hacky code in Element.cpp to handle
re-resolving.

The resolve-on-adoption behavior matches the behavior recently specced for
<img> elements in HTML (see http://html5.org/tools/web-apps-tracker?from=8508&to=8509)

This does not yet support any other form of url() in CSS (-webkit-image-set
being the most glaring omission). I've left FIXMEs in the code to handle those,
but I fear adding any more hacks in Element.cpp; in order to support this
generally, some serious refactoring in the CSSValue class hierarchy is required.

BUG=229142
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/164803002

git-svn-id: svn://svn.chromium.org/blink/trunk@167931 bbb929c8-8fbe-4397-9dbb-9b2b20218538