MemoryCache correctness improvements.

Cache validation previously wasn't checking the redirect chain for
most resource types. Nor was status code being used carefully
enough. This patch fixes that, and introduces a unit test that
confirms basic cache validating functionality.

R=japhet@chromium.org
BUG=103458

Review URL: https://codereview.chromium.org/145563004

git-svn-id: svn://svn.chromium.org/blink/trunk@166327 bbb929c8-8fbe-4397-9dbb-9b2b20218538