Pull up clearCacheExceptFrame to ImageDecoder.

third_party/WebKit/Source/platform/image-decoders/webp/WEBPImageDecoder.h

Index: third_party/WebKit/Source/platform/image-decoders/webp/WEBPImageDecoder.h
diff --git a/third_party/WebKit/Source/platform/image-decoders/webp/WEBPImageDecoder.h b/third_party/WebKit/Source/platform/image-decoders/webp/WEBPImageDecoder.h
index fde848af48feec88df6327db774f49fb8f0a0c52..8380ee4336613b4f4a05c853e7f336cd915bf414 100644
--- a/third_party/WebKit/Source/platform/image-decoders/webp/WEBPImageDecoder.h
+++ b/third_party/WebKit/Source/platform/image-decoders/webp/WEBPImageDecoder.h
@@ -50,7 +50,6 @@ class PLATFORM_EXPORT WEBPImageDecoder final : public ImageDecoder {
   int repetitionCount() const override;
   bool frameIsCompleteAtIndex(size_t) const override;
   float frameDurationAtIndex(size_t) const override;
-  size_t clearCacheExceptFrame(size_t) override;
 
  private:
   // ImageDecoder:
@@ -63,6 +62,17 @@ class PLATFORM_EXPORT WEBPImageDecoder final : public ImageDecoder {
                          size_t dataSize,
                          size_t frameIndex);
 
+  // For WebP images, the frame status needs to be FrameComplete to decode
+  // subsequent frames that depend on frame |index|. The reason for this is that
+  // WebP uses the previous frame for alpha blending, in applyPostProcessing().
+  //
+  // Before calling this, verify that frame |index| exists by checking that
+  // |index| is smaller than |m_frameBufferCache|.size().
+  bool frameStatusSufficientForSuccessors(size_t index) override {
+    DCHECK(index < m_frameBufferCache.size());
+    return m_frameBufferCache[index].getStatus() == ImageFrame::FrameComplete;
+  }
+
   WebPIDecoder* m_decoder;
   WebPDecBuffer m_decoderBuffer;
   int m_formatFlags;