Update 'WebURLRequest::RequestContext' to match Fetch.

Based on the conversation in [1], Fetch's request contexts have expanded
a bit[2]. This CL brings our implementation into line.

It should have zero practical impact, as we're not really using them for
anything useful yet. Working on that in a subsequence CL.

[1]: https://www.w3.org/Bugs/Public/show_bug.cgi?id=26247
[2]: http://fetch.spec.whatwg.org/#concept-request-context

BUG=390497

Review URL: https://codereview.chromium.org/386583003

git-svn-id: svn://svn.chromium.org/blink/trunk@177826 bbb929c8-8fbe-4397-9dbb-9b2b20218538