Move SubstituteData processing to ResourceFetcher

Use a technique similar to how data: images are processed. Namely, pre-populate a Resource and put it in the MemoryCache which can than be treated as a cache hit.

In the case of main resources, which we don't normally reuse, we allow the Resource created from the SubstituteData to live in the MemoryCache temporarily. When the "cache hit" occurs, we remove it to ensure it isn't actually reused.

BUG=

Review URL: https://codereview.chromium.org/118433003

git-svn-id: svn://svn.chromium.org/blink/trunk@164423 bbb929c8-8fbe-4397-9dbb-9b2b20218538