Migrate user agent string to Cow<'static, str>.

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.

components/servo/lib.rs

@@ -252,7 +252,7 @@ fn create_constellation(opts: opts::Opts,
     let bluetooth_thread: IpcSender<BluetoothMethodMsg> = BluetoothThreadFactory::new();
 
     let (public_resource_threads, private_resource_threads) =
-        new_resource_threads(opts.user_agent.clone(),
+        new_resource_threads(opts.user_agent,
                              devtools_chan.clone(),
                              time_profiler_chan.clone(),
                              opts.config_dir.map(Into::into));