cc: Clear checker-image tracking on navigation and visibility changes.

cc/tiles/checker_image_tracker.h

Index: cc/tiles/checker_image_tracker.h
diff --git a/cc/tiles/checker_image_tracker.h b/cc/tiles/checker_image_tracker.h
index db5ccf404594a38c5cede7ce3e683180e73a7be8..3a1ccf7614164a5c5c9f27dbbb97c84ad1493910 100644
--- a/cc/tiles/checker_image_tracker.h
+++ b/cc/tiles/checker_image_tracker.h
@@ -47,8 +47,14 @@ class CC_EXPORT CheckerImageTracker {
   // Returns the set of images to invalidate on the sync tree.
   const ImageIdFlatSet& TakeImagesToInvalidateOnSyncTree();
 
+  // Called when the sync tree is activated. Each call to
+  // TakeImagesToInvalidateOnSyncTree() must be followed by this when the
+  // invalidated sync tree is activated.
   void DidActivateSyncTree();
 
+  // Called to reset the tracker state on navigation or when going invisible.
+  void ClearTracker();
+
  private:
   enum class DecodePolicy {
     // The image can be decoded asynchronously from raster. When set, the image
@@ -62,6 +68,20 @@ class CC_EXPORT CheckerImageTracker {
     SYNC_PERMANENT,
   };
 
+  class ScopedDecodeHolder {

[vmpstr, 2017/05/09 22:06:21]

ScopedDecodeUnlocker maybe? Can you also drop a class comment here to explain
what it does

[Khushal, 2017/05/10 22:47:09]

Done.

+   public:
+    ScopedDecodeHolder(ImageController* controller,
+                       ImageController::ImageDecodeRequestId request_id)
+        : controller_(controller), request_id_(request_id) {}
+    ~ScopedDecodeHolder() { controller_->UnlockImageDecode(request_id_); }
+
+   private:
+    ImageController* controller_;
+    ImageController::ImageDecodeRequestId request_id_;
+
+    DISALLOW_COPY_AND_ASSIGN(ScopedDecodeHolder);
+  };
+
   void DidFinishImageDecode(ImageId image_id,
                             ImageController::ImageDecodeRequestId request_id,
                             ImageController::ImageDecodeResult result);
@@ -91,14 +111,11 @@ class CC_EXPORT CheckerImageTracker {
   sk_sp<const SkImage> outstanding_image_decode_;
 
   // A map of ImageId to its DecodePolicy.
-  // TODO(khushalsagar): Limit the size of this set.
-  // TODO(khushalsagar): Plumb navigation changes here to reset this. See
-  // crbug.com/693228.
   std::unordered_map<ImageId, DecodePolicy> image_async_decode_state_;
 
   // A map of image id to image decode request id for images to be unlocked.
-  std::unordered_map<ImageId, ImageController::ImageDecodeRequestId>
-      image_id_to_decode_request_id_;
+  std::unordered_map<ImageId, std::unique_ptr<ScopedDecodeHolder>>
+      image_id_to_decode_;
 
   base::WeakPtrFactory<CheckerImageTracker> weak_factory_;
 };