[CSS Shapes] Shape images are now image types, not just URIs

This change supports potentially-CORS-enabled loading of image-set elements by shape-inside and shape-outside.

The changes to CSSImageSetValue are analogous to the ones I made in https://codereview.chromium.org/40653002 for CSSImageValue.

The StyleResourceLoader change is slightly awkward to preserve StyleResourceLoader.h's low-impact #include list. I didn't make doLoadPendingImage() (nee loadPendingImage) a private method to avoid the dependency on the CORSEnabled enum, which is defined in ResourceFetcher.h.

BUG=328180

Review URL: https://codereview.chromium.org/128623004

git-svn-id: svn://svn.chromium.org/blink/trunk@164993 bbb929c8-8fbe-4397-9dbb-9b2b20218538