Add file backend support for Blob and related

Changes include:
- Add BlobImpl to Blob, and related caching mechanism
- Expose ResourceThreads to document_loader, workerglobalscope, worker, and global
- Fix encode_multipart_form_data
- Other small fixes to accommodate the above changes

components/script/dom/bindings/global.rs

@@ -19,7 +19,8 @@ use js::jsapi::{CurrentGlobalOrNull, GetGlobalForObjectCrossCompartment};
 use js::jsapi::{JSContext, JSObject, JS_GetClass, MutableHandleValue};
 use js::{JSCLASS_IS_DOMJSCLASS, JSCLASS_IS_GLOBAL};
 use msg::constellation_msg::{PipelineId, PanicMsg};
-use net_traits::{CoreResourceThread, RequestSource};
+use net_traits::filemanager_thread::FileManagerThreadMsg;
+use net_traits::{ResourceThreads, CoreResourceThread, RequestSource, IpcSend};
 use profile_traits::{mem, time};
 use script_runtime::{CommonScriptMsg, ScriptChan, ScriptPort};
 use script_thread::{MainThreadScriptChan, ScriptThread};
@@ -122,19 +123,29 @@ impl<'a> GlobalRef<'a> {
         }
     }
 
-    /// Get the `CoreResourceThread` for this global scope.
-    pub fn core_resource_thread(&self) -> CoreResourceThread {
+    /// Get the `ResourceThreads` for this global scope.
+    pub fn resource_threads(&self) -> ResourceThreads {
         match *self {
             GlobalRef::Window(ref window) => {
                 let doc = window.Document();
                 let doc = doc.r();
                 let loader = doc.loader();
-                (*loader.resource_thread).clone()
+                loader.resource_threads().clone()
             }
-            GlobalRef::Worker(ref worker) => worker.core_resource_thread().clone(),
+            GlobalRef::Worker(ref worker) => worker.resource_threads().clone(),
         }
     }
 
+    /// Get the `CoreResourceThread` for this global scope
+    pub fn core_resource_thread(&self) -> CoreResourceThread {
+        self.resource_threads().sender()
+    }
+
+    /// Get the port to file manager for this global scope
+    pub fn filemanager_thread(&self) -> IpcSender<FileManagerThreadMsg> {
+        self.resource_threads().sender()
+    }
+
     /// Get the worker's id.
     pub fn get_worker_id(&self) -> Option<WorkerId> {
         match *self {