Move Data uri handling into WebCore

Move Data uri handling into WebCore

** this patch is not yet ready for review **

This patch moves data url handling to WebCore from its previous
home in in weburlloader_impl.cc.

SVG images are sandboxed to prevent network loads (images loading
other images) but we should whitelist data uris. Passing a null
networking context into weburlloader has been a source of security
bugs in the past.

This patch hooks data uri handling in ResourceLoader and uses
the substitute resource architecture to deliver data uri resources.

BUG=224317