Move inval dedup to Window for wider usages.

tools/viewer/sk_app/Window.h

Index: tools/viewer/sk_app/Window.h
diff --git a/tools/viewer/sk_app/Window.h b/tools/viewer/sk_app/Window.h
index 63d5e19e5594716c9bc95742b0fac51fe8b9af75..c1718c44ff73de58e1dc3056ead05bd2ff164479 100644
--- a/tools/viewer/sk_app/Window.h
+++ b/tools/viewer/sk_app/Window.h
@@ -12,6 +12,7 @@
 #include "SkRect.h"
 #include "SkTouchGesture.h"
 #include "SkTypes.h"
+#include "../private/SkMutex.h"
 
 class SkCanvas;
 
@@ -27,8 +28,15 @@ public:
 
     virtual void setTitle(const char*) = 0;
     virtual void show() = 0;
+
+    // Make sure that onPaint is called in the subclasses' implementations of inval.
+    // We will unset fIsContentInvalided in onPaint, which allows checkAndInval
+    // to invalidate the content after the onPaint.
     virtual void inval() = 0;
 
+    // Check if the current window has any pending inval; call inval if not.
+    void checkAndInval();
+
     virtual bool scaleContentToFit() const { return false; }
     virtual bool supportsContentRect() const { return false; }
     virtual SkRect getContentRect() { return SkRect::MakeEmpty(); }
@@ -164,6 +172,9 @@ protected:
     void*        fPaintUserData;
 
     WindowContext* fWindowContext = nullptr;
+
+    bool fIsContentInvalidated = false;  // use this to avoid duplicate invalidate events
+    SkMutex fMutex;
 };
 
 }   // namespace sk_app