Delete dead invalidate function.

sky/engine/public/platform/WebLayer.h

Index: sky/engine/public/platform/WebLayer.h
diff --git a/sky/engine/public/platform/WebLayer.h b/sky/engine/public/platform/WebLayer.h
index 2f2200f74153bce49b560cf8e6883dcf1171d9ed..7398c07fc6f686a0e4cf0eaf497300d00686719d 100644
--- a/sky/engine/public/platform/WebLayer.h
+++ b/sky/engine/public/platform/WebLayer.h
@@ -55,9 +55,6 @@ public:
     // Returns a positive ID that will be unique across all WebLayers allocated in this process.
     virtual int id() const = 0;
 
-    // Sets the entire layer as invalid, i.e. needs to update its content.
-    virtual void invalidate() = 0;
-
     // These functions do not take ownership of the WebLayer* parameter.
     virtual void addChild(WebLayer*) = 0;
     virtual void insertChild(WebLayer*, size_t index) = 0;