Allow WebExternalTextureLayerClient to work with mailboxes.

cc/layers/texture_layer_client.h

Index: cc/layers/texture_layer_client.h
diff --git a/cc/layers/texture_layer_client.h b/cc/layers/texture_layer_client.h
index 39f1e80d4cb6997db6445379e205090b71b2c56b..b5c0de61045086add6a5581d52f88c2b80c65608 100644
--- a/cc/layers/texture_layer_client.h
+++ b/cc/layers/texture_layer_client.h
@@ -9,6 +9,7 @@ namespace WebKit { class WebGraphicsContext3D; }
 
 namespace cc {
 class ResourceUpdateQueue;
+class TextureMailbox;
 
 class TextureLayerClient {
  public:
@@ -21,6 +22,11 @@ class TextureLayerClient {
   // and detecting lost context.
   virtual WebKit::WebGraphicsContext3D* Context3d() = 0;
 
+  // Returns true and provides a mailbox if a new frame is available.
+  // Returns false if no new data is available
+  // and the old mailbox is to be reused.
+  virtual bool PrepareTextureMailbox(TextureMailbox* mailbox) = 0;
+
  protected:
   virtual ~TextureLayerClient() {}
 };