Make non-initial about:blank loads asynchronous

Don't update iframe pipeline until load completes

To preserve the previous functionality of delaying load events when a
new navigation is triggered, pending pipeline id represents the
current pending load. The load event is only fired if the load message's
pipeline id matches the pending pipeline id.

Track frame size on Frame instead of Pipeline

Disabled matchMedia test

Track creator pipeline id

components/constellation/pipeline.rs

@@ -76,10 +76,6 @@ pub struct Pipeline {
     /// The title of the most recently-loaded page.
     pub title: Option<String>,
 
-    /// The size of the frame.
-    /// TODO: move this field to `Frame`.
-    pub size: Option<TypedSize2D<f32, CSSPixel>>,
-
     /// Whether this pipeline is currently running animations. Pipelines that are running
     /// animations cause composites to be continually scheduled.
     pub running_animations: bool,
@@ -291,7 +287,6 @@ impl Pipeline {
                          state.compositor_proxy,
                          state.is_private,
                          url,
-                         state.window_size,
                          state.prev_visibility.unwrap_or(true)))
     }
 
@@ -305,7 +300,6 @@ impl Pipeline {
                compositor_proxy: Box<CompositorProxy + 'static + Send>,
                is_private: bool,
                url: ServoUrl,
-               size: Option<TypedSize2D<f32, CSSPixel>>,
                visible: bool)
                -> Pipeline {
         let pipeline = Pipeline {
@@ -318,7 +312,6 @@ impl Pipeline {
             url: url,
             title: None,
             children: vec!(),
-            size: size,
             running_animations: false,
             visible: visible,
             is_private: is_private,