[blink] Add a ScriptCachedMetadataHandler class.

- Moves ScriptResource::SingleCachedMetadataHandlerImpl out of
  ScriptResource and into the blink namespace so it can be used by other
  script-like resources.
- Creates this CacheHandler type for RawResource type kRaw, which will
  be needed for WASM modules.

Bug: chromium:719172
Change-Id: I34a2740310d2d3f4ccb0cea1bb664db0b9c53f5e
Reviewed-on: https://chromium-review.googlesource.com/1174903
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584159}
7 files changed