cc: Remove cc::Thread and cc::ThreadImpl.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index ad37ab731dcc338e369d05b877ff5bdc40886302..ed3889d556dea942cc81b5bda9889f6c0e0ac0a3 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -14,7 +14,6 @@
 #include "cc/animation/scrollbar_animation_controller.h"
 #include "cc/animation/timing_function.h"
 #include "cc/base/math_util.h"
-#include "cc/base/thread.h"
 #include "cc/base/util.h"
 #include "cc/debug/debug_rect_history.h"
 #include "cc/debug/frame_rate_counter.h"
@@ -767,6 +766,8 @@ void LayerTreeHostImpl::MainThreadHasStoppedFlinging() {
 
 void LayerTreeHostImpl::UpdateBackgroundAnimateTicking(
     bool should_background_tick) {
+  DCHECK(proxy_->IsImplThread());
+
   bool enabled = should_background_tick &&
                  !animation_registrar_->active_animation_controllers().empty();
 
@@ -776,7 +777,7 @@ void LayerTreeHostImpl::UpdateBackgroundAnimateTicking(
     time_source_client_adapter_ = LayerTreeHostImplTimeSourceAdapter::Create(
         this,
         DelayBasedTimeSource::Create(LowFrequencyAnimationInterval(),
-                                     proxy_->CurrentThread()->TaskRunner()));
+                                     proxy_->ImplThreadTaskRunner()));
   }
 
   time_source_client_adapter_->SetActive(enabled);