Reland again "Decode GIF image frames on demand".

Source/core/platform/image-decoders/skia/ImageDecoderSkia.cpp

Index: Source/core/platform/image-decoders/skia/ImageDecoderSkia.cpp
diff --git a/Source/core/platform/image-decoders/skia/ImageDecoderSkia.cpp b/Source/core/platform/image-decoders/skia/ImageDecoderSkia.cpp
index f93bf34faa0085f9ecab4b7f50c87d053afadf64..0c49900f7b32dbc4377a49292fd283fed5f5bc5d 100644
--- a/Source/core/platform/image-decoders/skia/ImageDecoderSkia.cpp
+++ b/Source/core/platform/image-decoders/skia/ImageDecoderSkia.cpp
@@ -40,6 +40,10 @@ ImageFrame::ImageFrame()
     , m_duration(0)
     , m_disposalMethod(DisposeNotSpecified)
     , m_premultiplyAlpha(true)
+    , m_requiredPreviousFrameIndex(notFound)
+#if !ASSERT_DISABLED
+    , m_requiredPreviousFrameIndexValid(false)
+#endif
 {
 }
 
@@ -61,6 +65,11 @@ ImageFrame& ImageFrame::operator=(const ImageFrame& other)
     // Be sure that this is called after we've called setStatus(), since we
     // look at our status to know what to do with the alpha value.
     setHasAlpha(other.hasAlpha());
+    // Copy raw fields to avoid ASSERT failure in requiredPreviousFrameIndex().
+    m_requiredPreviousFrameIndex = other.m_requiredPreviousFrameIndex;
+#if !ASSERT_DISABLED
+    m_requiredPreviousFrameIndexValid = other.m_requiredPreviousFrameIndexValid;
+#endif
     return *this;
 }