Fix forward declarations

Fix forward declarations in cc/animation/

R=vollick@chromium.org

Review URL: https://codereview.chromium.org/826453002

Cr-Commit-Position: refs/heads/master@{#309570}

cc/animation/layer_animation_event_observer.h

@@ -9,6 +9,8 @@
 
 namespace cc {
 
+struct AnimationEvent;
+
 class CC_EXPORT LayerAnimationEventObserver {
  public:
   virtual void OnAnimationStarted(const AnimationEvent& event) = 0;

cc/animation/layer_animation_value_observer.h

@@ -7,9 +7,14 @@
 
 #include "cc/base/cc_export.h"
 
+namespace gfx {
+class ScrollOffset;
+class Transform;
+}
+
 namespace cc {
 
-class ScrollOffset;
+class FilterOperations;
 
 class CC_EXPORT LayerAnimationValueObserver {
  public:

cc/animation/layer_animation_value_provider.h

@@ -7,9 +7,11 @@
 
 #include "cc/base/cc_export.h"
 
-namespace cc {
-
+namespace gfx {
 class ScrollOffset;
+}
+
+namespace cc {
 
 // A LayerAnimationValueProvider is used for determining the starting value
 // for animations that start at their 'current' value rather than at a