Teach ContentSecurityPolicy about WebURLRequest::RequestContext.

Rather than using `Resource::Type` to make decisions about what CSP
directive to apply to a resource request, we should be using
WebURLRequest::RequestContext. This patch teaches ContentSecurityPolicy
to correctly compare a URL against the right policy directive when given
a 'ResourceContext', and pipes the value through
'ResourceFetcher::canRequest' to make sure its available when it needs
to be.

BUG=390497

Review URL: https://codereview.chromium.org/398313002

git-svn-id: svn://svn.chromium.org/blink/trunk@178571 bbb929c8-8fbe-4397-9dbb-9b2b20218538