cc/blimp: Set up hooks for scroll/scale sync.

cc/blimp/remote_compositor_bridge_client.h

Index: cc/blimp/remote_compositor_bridge_client.h
diff --git a/cc/blimp/remote_compositor_bridge_client.h b/cc/blimp/remote_compositor_bridge_client.h
index 434dd0abfae24bbc7a14c9148176e23236ddeb37..1df4e0af2f52d39fc214e7afc3d0a7606cab437f 100644
--- a/cc/blimp/remote_compositor_bridge_client.h
+++ b/cc/blimp/remote_compositor_bridge_client.h
@@ -5,6 +5,8 @@
 #ifndef CC_BLIMP_REMOTE_COMPOSITOR_BRIDGE_CLIENT_H_
 #define CC_BLIMP_REMOTE_COMPOSITOR_BRIDGE_CLIENT_H_
 
+#include <unordered_map>
+
 #include "base/macros.h"
 #include "cc/base/cc_export.h"
 
@@ -12,11 +14,17 @@ namespace base {
 class SingleThreadTaskRunner;
 }  // namespace base
 
+namespace gfx {
+class ScrollOffset;
+}  // namespace gfx
+
 namespace cc {
 class CompositorProtoState;
 
 class CC_EXPORT RemoteCompositorBridgeClient {
  public:
+  using ScrollOffsetMap = std::unordered_map<int, gfx::ScrollOffset>;
+
   virtual ~RemoteCompositorBridgeClient() {}
 
   // Called in response to a ScheduleMainFrame request made on the
@@ -24,6 +32,14 @@ class CC_EXPORT RemoteCompositorBridgeClient {
   // Note: The method should always be invoked asynchronously after the request
   // is made.
   virtual void BeginMainFrame() = 0;
+
+  // Provides an update from the mutations made on the client. Returns true if
+  // the update could be successfully applied to the engine state. This can
+  // fail, for instance, if the layer present in the update was destroyed on the
+  // engine.
+  virtual bool ApplyScrollAndScaleUpdateFromClient(
+      const ScrollOffsetMap& client_scroll_map,
+      float client_page_scale) = 0;
 };
 
 }  // namespace cc