Don't overwrit image data case entries that are just marked usable.

ppapi/proxy/ppb_image_data_proxy.h

Index: ppapi/proxy/ppb_image_data_proxy.h
===================================================================
--- ppapi/proxy/ppb_image_data_proxy.h	(revision 153070)
+++ ppapi/proxy/ppb_image_data_proxy.h	(working copy)
@@ -62,9 +62,12 @@
 
   const PP_ImageDataDesc& desc() const { return desc_; }
 
-  // Fills the contents of the image with 0.
-  void ZeroContents();
+  void set_used_in_replace_contents() { used_in_replace_contents_ = true; }
 
+  // Prepares this image data to be recycled to the plugin. The contents will be
+  // cleared if zero_contents is set.
+  void RecycleToPlugin(bool zero_contents);
+
 #if !defined(OS_NACL)
   static ImageHandle NullHandle();
   static ImageHandle HandleFromInt(int32_t i);
@@ -84,6 +87,10 @@
   scoped_ptr<skia::PlatformCanvas> mapped_canvas_;
 #endif
 
+  // Set to true when this ImageData has been used in a call to
+  // Graphics2D.ReplaceContents. This is used to signal that it can be cached.
+  bool used_in_replace_contents_;
+
   DISALLOW_COPY_AND_ASSIGN(ImageData);
 };