This commit allows headers with "content-type" name to be classified as valid header name, depending on its value according to [the Fetch spec](https://fetch.spec.whatwg.org/#cors-safelisted-request-header). As a result of this change, more request web platform tests pass, whose expected test results are updated with this commit.
This commit adds new files related to implementing the [Request
API](https://fetch.spec.whatwg.org/#request-class). This commit also
changes the expected web platform tests results. It also modifies the
following files:
components/net_traits/request.rs
HeapSizeOf is implemented in net_traits/request so that dom::request can
be used as a wrapper around net_traits::request::Request.
components/script/dom/headers.rs
Several methods are added to Headers so that request can access and
modify some of the headers fields.
