remove FetchContext::getCachePolicy()

CachePolicy represents per-frame memory cache policy, and used to
determine WebCachePolicy for resourceRequestCachePolicy in
FrameFetchContext.

Since the last caller of getCachePolicy() is only in FrameFetchContext,
remove the virtual method completely, and replace it with some
anonymous functions.

BUG=602900

Review-Url: https://codereview.chromium.org/1867493003
Cr-Commit-Position: refs/heads/master@{#458708}
5 files changed