Refactored image cache task - details below.

* Simpler image cache API for clients to use.
 * Significantly fewer threads.
   * One thread for image cache task (multiplexes commands, decoder threads and async resource requests).
   * 4 threads for decoder worker tasks.
 * Removed ReflowEvent hacks in script and layout tasks.
   * Image elements pass a Trusted<T> to image cache, which is used to dirty nodes via script task. Previous use of Untrusted addresses was unsafe.
   * Image requests such as background-image on layout / paint threads trigger repaint only rather than full reflow.
 * Add reflow batching for when multiple images load quickly.
   * Reduces the number of paints loading wikipedia from ~95 to ~35.
 * Reasonably simple to add proper prefetch support in a follow up PR.
 * Async loaded images always construct Image fragments now, instead of generic.
   * Image fragments support the image not being present.
 * Simpler implementation of synchronous image loading for reftests.
 * Removed image holder.
 * image.onload support.
 * image NaturalWidth and NaturalHeight support.
 * Updated WPT expectations.

components/layout_traits/lib.rs

@@ -50,7 +50,7 @@ pub trait LayoutTaskFactory {
               script_chan: ScriptControlChan,
               paint_chan: PaintChan,
               resource_task: ResourceTask,
-              img_cache_task: ImageCacheTask,
+              image_cache_task: ImageCacheTask,
               font_cache_task: FontCacheTask,
               time_profiler_chan: time::ProfilerChan,
               mem_profiler_chan: mem::ProfilerChan,