In most scenarios, where the user of Servo will not override the default
user agent, the user agent can be a `&'static str`. But since we allow
for customization, we currently use a `String` to represent the user
agent. This commit migrates the user agent to be represented as a
`Cow<'static, str>`, which (at the cost of ergonomics) prevents
unnecessary allocations whenever cloning the user agent string in the
scenario the user doesn't override the user agent.
Changes include:
- Introduce an IpcSend trait to abstract over a collection of IpcSenders
- Implement ResourceThreads collection to abstract the resource-related
sub threads across the component
- Rename original ResourceThread and ControlMsg into an unifed CoreResource__
to accommodate above changes and avoid confusions
add pass-through from doc to http-loader for referrer_policy, ref_URL
add logic for setting referer header
add script pass-through for referrer
add unit tests for setting referer header
In rust-url 1.0 the `Url` struct is going to have private fields, and there
is no way to to create an aribitrary one without going through the parser.
The plugin never had a clear demonstrated performance benefit,
it was made mostly because it was possible and relatively easy at the time.
