[Code cleanup] Change the order of forward-declaration in LTHI

The forward declaration in in layer_tree_host_impl.h seems to be in
alphabetic order where one of them isn't.

This CL corrects the order, and also removes a forward-declaration for
a struct which isn't needed.

Bug: None
Change-Id: I275a1af8392f20ec023a69107aa508b2a29b3d55
Reviewed-on: https://chromium-review.googlesource.com/c/1352515Reviewed-by: weiliangc <weiliangc@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611538}

cc/trees/layer_tree_host_impl.h

@@ -67,12 +67,12 @@ class CompositorFrameMetadata;
 
 namespace cc {
 class BrowserControlsOffsetManager;
-class LayerTreeFrameSink;
 class DebugRectHistory;
 class EvictionTilePriorityQueue;
 class FrameRateCounter;
 class ImageAnimationController;
 class LayerImpl;
+class LayerTreeFrameSink;
 class LayerTreeImpl;
 class MemoryHistory;
 class MutatorEvents;
@@ -91,7 +91,6 @@ class SwapPromiseMonitor;
 class SynchronousTaskGraphRunner;
 class TaskGraphRunner;
 class UIResourceBitmap;
-struct ScrollAndScaleSet;
 class Viewport;
 
 using BeginFrameCallbackList = std::vector<base::Closure>;