Early reporting of opaqueness by decoders.

third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp

Index: third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp
diff --git a/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp b/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp
index 824764c5534764548b2c1eaa4225592de2b26d5f..8889cc5af3554430a9028e000435c8f29c18bd22 100644
--- a/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp
+++ b/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp
@@ -88,6 +88,19 @@ bool GIFImageDecoder::frameIsCompleteAtIndex(size_t index) const
     return m_reader && (index < m_reader->imagesCount()) && m_reader->frameContext(index)->isComplete();
 }
 
+bool GIFImageDecoder::frameHasAlphaAtIndex(size_t index) const
+{
+    if (!frameIsCompleteAtIndex(index))
+        return true;
+    if (m_reader) {
+        const GIFFrameContext& frameContext = *m_reader->frameContext(index);
+        const GIFColorMap::Table& colorTable = frameContext.localColorMap().isDefined() ? frameContext.localColorMap().getTable() : m_reader->globalColorMap().getTable();
+        if (frameContext.transparentPixel() >= colorTable.size())

[scroggo_chromium, 2016/05/16 20:38:20]

Isn't it also possible that this frame does not cover the frame behind it, which
may have transparency? (Or, for that matter, could it have no frames behind it?
Wouldn't that also result in transparency?)

+            return false;
+    }
+    return m_frameBufferCache[index].hasAlpha();
+}
+
 float GIFImageDecoder::frameDurationAtIndex(size_t index) const
 {
     return (m_reader && (index < m_reader->imagesCount()) &&