Don't ignore Accept: header for cache/XHR dedupe purposes

See http://crbug.com/315939; varying the Accept header can vary the
response we get, so we shouldn't dedupe/cache based on it.

BUG=chromium:315939

Review URL: https://codereview.chromium.org/85623006

git-svn-id: svn://svn.chromium.org/blink/trunk@162639 bbb929c8-8fbe-4397-9dbb-9b2b20218538