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/net/fetch/methods.rs

@@ -29,6 +29,7 @@ use net_traits::request::{Type, Origin, Window};
 use net_traits::response::{HttpsState, TerminationReason};
 use net_traits::response::{Response, ResponseBody, ResponseType};
 use resource_thread::CancellationListener;
+use std::borrow::Cow;
 use std::collections::HashSet;
 use std::fs::File;
 use std::io::Read;
@@ -51,7 +52,7 @@ enum Data {
 
 pub struct FetchContext {
     pub state: HttpState,
-    pub user_agent: String,
+    pub user_agent: Cow<'static, str>,
     pub devtools_chan: Option<Sender<DevtoolsControlMsg>>,
 }
 
@@ -763,7 +764,8 @@ fn http_network_or_cache_fetch(request: Rc<Request>,
 
     // Step 8
     if !http_request.headers.borrow().has::<UserAgent>() {
-        http_request.headers.borrow_mut().set(UserAgent(context.user_agent.clone()));
+        let user_agent = context.user_agent.clone().into_owned();
+        http_request.headers.borrow_mut().set(UserAgent(user_agent));
     }
 
     match http_request.cache_mode.get() {

components/net/http_loader.rs

@@ -39,7 +39,7 @@ use openssl::ssl::error::{OpensslError, SslError};
 use profile_traits::time::{ProfilerCategory, ProfilerChan, TimerMetadata, profile};
 use profile_traits::time::{TimerMetadataFrameType, TimerMetadataReflowType};
 use resource_thread::{AuthCache, AuthCacheEntry, CancellationListener, send_error, start_sending_sniffed_opt};
-use std::borrow::ToOwned;
+use std::borrow::{Cow, ToOwned};
 use std::boxed::FnBox;
 use std::collections::HashSet;
 use std::error::Error;
@@ -57,7 +57,7 @@ use util::prefs::PREFS;
 use util::thread::spawn_named;
 use uuid;
 
-pub fn factory(user_agent: String,
+pub fn factory(user_agent: Cow<'static, str>,
                http_state: HttpState,
                devtools_chan: Option<Sender<DevtoolsControlMsg>>,
                profiler_chan: ProfilerChan,
@@ -137,7 +137,7 @@ fn load_for_consumer(load_data: LoadData,
                      devtools_chan: Option<Sender<DevtoolsControlMsg>>,
                      swmanager_chan: Option<IpcSender<CustomResponseMediator>>,
                      cancel_listener: CancellationListener,
-                     user_agent: String) {
+                     user_agent: Cow<'static, str>) {
     let factory = NetworkHttpRequestFactory {
         connector: connector,
     };
@@ -865,7 +865,7 @@ pub fn load<A, B>(load_data: &LoadData,
                   http_state: &HttpState,
                   devtools_chan: Option<Sender<DevtoolsControlMsg>>,
                   request_factory: &HttpRequestFactory<R=A>,
-                  user_agent: String,
+                  user_agent: Cow<'static, str>,
                   cancel_listener: &CancellationListener,
                   swmanager_chan: Option<IpcSender<CustomResponseMediator>>)
                   -> Result<StreamedResponse, LoadError> where A: HttpRequest + 'static, B: UIProvider {

components/net/resource_thread.rs

@@ -35,7 +35,7 @@ use net_traits::storage_thread::StorageThreadMsg;
 use profile_traits::time::ProfilerChan;
 use rustc_serialize::{Decodable, Encodable};
 use rustc_serialize::json;
-use std::borrow::ToOwned;
+use std::borrow::{Cow, ToOwned};
 use std::boxed::FnBox;
 use std::cell::Cell;
 use std::collections::HashMap;
@@ -164,7 +164,7 @@ fn start_sending_opt(start_chan: LoadConsumer, metadata: Metadata,
 }
 
 /// Returns a tuple of (public, private) senders to the new threads.
-pub fn new_resource_threads(user_agent: String,
+pub fn new_resource_threads(user_agent: Cow<'static, str>,
                             devtools_chan: Option<Sender<DevtoolsControlMsg>>,
                             profiler_chan: ProfilerChan,
                             config_dir: Option<PathBuf>)
@@ -181,7 +181,7 @@ pub fn new_resource_threads(user_agent: String,
 
 
 /// Create a CoreResourceThread
-pub fn new_core_resource_thread(user_agent: String,
+pub fn new_core_resource_thread(user_agent: Cow<'static, str>,
                                 devtools_chan: Option<Sender<DevtoolsControlMsg>>,
                                 profiler_chan: ProfilerChan,
                                 config_dir: Option<PathBuf>)
@@ -470,7 +470,7 @@ pub struct AuthCache {
 }
 
 pub struct CoreResourceManager {
-    user_agent: String,
+    user_agent: Cow<'static, str>,
     mime_classifier: Arc<MimeClassifier>,
     devtools_chan: Option<Sender<DevtoolsControlMsg>>,
     swmanager_chan: Option<IpcSender<CustomResponseMediator>>,
@@ -481,7 +481,7 @@ pub struct CoreResourceManager {
 }
 
 impl CoreResourceManager {
-    pub fn new(user_agent: String,
+    pub fn new(user_agent: Cow<'static, str>,
                devtools_channel: Option<Sender<DevtoolsControlMsg>>,
                profiler_chan: ProfilerChan) -> CoreResourceManager {
         CoreResourceManager {