Avoid reusing XMLHttpRequest resources during document load.

The determineRevalidationPolicy() predicate decides how an existing
cached resource can be (re)utilized upon another fetch to the same
resource. It will allow reuse of already loaded (and loading)
resources while a document is being loaded.

Such resource reuse isn't safe to do for raw resources requested
through XHR, as there's too much state and header information that an
XHR request can be furnished with. Hence, we exempt raw resources and
delegate to the standard cache control policy to decide on reuse. Like
it would do once the document has loaded.

R=japhet@chromium.org
BUG=314433

Review URL: https://codereview.chromium.org/59043006

git-svn-id: svn://svn.chromium.org/blink/trunk@161846 bbb929c8-8fbe-4397-9dbb-9b2b20218538