Teach WebURLRequest about Fetch's "frame type" concept.

This CL introduces a new enum property on resource requests which maps
to the "frame type" concept in Fetch[1]. The eventual goal is to migrate
mixed content and CSP checks which rely on knowledge of a request's
browsing context to use this new property rather than overloading the
request context with information about the request's browsing context
and originator.

To ensure that we can rely on the property's value, we ASSERT that the
value is properly set when loading "main" resources.

[1]: http://fetch.spec.whatwg.org/#concept-request-context-frame-type

BUG=390497

Review URL: https://codereview.chromium.org/389993002

git-svn-id: svn://svn.chromium.org/blink/trunk@178069 bbb929c8-8fbe-4397-9dbb-9b2b20218538