Simplify determining CachePolicy

Merge the subresource and main resource paths and put them in FrameFetchContext.
They don't really need to be separate as far as I can tell, and it's a moot
point at the moment anyway, since we never cache main resources.

Remove CachePolicyCache, which is unused.

Remove the CachePolicy parameter from Resource::mustRevalidateDueToCacheHeaders.
The usage of CachePolicy in that function is simple enough to inline in
ResourceFetcher, and it means the function name is misleading, since we are
currently making a decision based on more than just the cache headers.

BUG=none

Review URL: https://codereview.chromium.org/114523004

git-svn-id: svn://svn.chromium.org/blink/trunk@163878 bbb929c8-8fbe-4397-9dbb-9b2b20218538