Pull up clearCacheExceptFrame to ImageDecoder.

third_party/WebKit/Source/platform/image-decoders/ImageDecoder.h

Index: third_party/WebKit/Source/platform/image-decoders/ImageDecoder.h
diff --git a/third_party/WebKit/Source/platform/image-decoders/ImageDecoder.h b/third_party/WebKit/Source/platform/image-decoders/ImageDecoder.h
index cb8ccebc0d91dd6a8319b1342863ece1bd302189..c5dd9532fdea684918ebabd03d40b07abeb0619d 100644
--- a/third_party/WebKit/Source/platform/image-decoders/ImageDecoder.h
+++ b/third_party/WebKit/Source/platform/image-decoders/ImageDecoder.h
@@ -240,11 +240,13 @@ class PLATFORM_EXPORT ImageDecoder {
 
   bool failed() const { return m_failed; }
 
-  // Clears decoded pixel data from all frames except the provided frame.
+  // Clears decoded pixel data from all frames except the provided frame. If
+  // subsequent frames depend on this frame's required previous frame, then that
+  // frame is also kept in cache to prevent re-decoding from the beginning.
   // Callers may pass WTF::kNotFound to clear all frames.
   // Note: If |m_frameBufferCache| contains only one frame, it won't be cleared.
   // Returns the number of bytes of frame data actually cleared.
-  virtual size_t clearCacheExceptFrame(size_t);
+  size_t clearCacheExceptFrame(size_t);
 
   // If the image has a cursor hot-spot, stores it in the argument
   // and returns true. Otherwise returns false.
@@ -343,6 +345,26 @@ class PLATFORM_EXPORT ImageDecoder {
   // future decodes to purge old frames as it goes.
   void updateAggressivePurging(size_t index);
 
+  // The method is only relevant for multi-frame images.
+  //
+  // This method indicates whether the provided frame has enough data to decode
+  // successive frames that depend on it. It is used by clearCacheExceptFrame
+  // to determine which frame to keep in cache when the indicated frame is not
+  // yet sufficiently decoded.
+  //
+  // The default condition is that the frame status needs to be FramePartial or
+  // FrameComplete, since the data of previous frames is copied in
+  // initFrameBuffer() before setting the status to FramePartial. For WebP,
+  // however, the status needs to be FrameComplete since the complete buffer is
+  // used to do alpha blending in WEBPImageDecoder::applyPostProcessing().
+  //
+  // Before calling this, verify that frame |index| exists by checking that
+  // |index| is smaller than |m_frameBufferCache|.size().
+  virtual bool frameStatusSufficientForSuccessors(size_t index) {
+    DCHECK(index < m_frameBufferCache.size());
+    return m_frameBufferCache[index].getStatus() != ImageFrame::FrameEmpty;
+  }
+
  private:
   enum class SniffResult { JPEG, PNG, GIF, WEBP, ICO, BMP, Invalid };