Improve GIF decoding performance

Source/core/platform/image-decoders/gif/GIFImageDecoderTest.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 433 matching lines @@
     EXPECT_EQ(frameCount, decoder->frameCount());
     ImageFrame* lastFrame = decoder->frameBufferAtIndex(frameCount - 1);
     EXPECT_EQ(baselineHashes[frameCount - 1], hashSkBitmap(lastFrame->getSkBitmap()));
     decoder->clearCacheExceptFrame(notFound);
 
     // Resume decoding of the first frame.
     ImageFrame* firstFrame = decoder->frameBufferAtIndex(0);
     EXPECT_EQ(ImageFrame::FrameComplete, firstFrame->status());
     EXPECT_EQ(baselineHashes[0], hashSkBitmap(firstFrame->getSkBitmap()));
 }
+
+// The first LZW code in the image is invalid. Decoding should fail.

[Peter Kasting, 2013/09/05 06:13:01]

Nit: Perhaps:

The first LZW codes in the image are invalid values that try to create a loop in
the dictionary.  Decoding should fail, but not infinitely loop or corrupt
memory.

(And something similar below)

[Alpha Left Google, 2013/09/05 17:02:28]

Done.

+TEST(GIFImageDecoderTest, badInitialCode)
+{
+    RefPtr<SharedBuffer> testData = readFile("/Source/web/tests/data/bad-initial-code.gif");
+    ASSERT_TRUE(testData.get());
+
+    OwnPtr<GIFImageDecoder> testDecoder(createDecoder());
+    testDecoder->setData(testData.get(), true);
+    EXPECT_EQ(1u, testDecoder->frameCount());
+    ASSERT_TRUE(testDecoder->frameBufferAtIndex(0));
+    EXPECT_TRUE(testDecoder->failed());
+}
+
+// Image has an invalid LZW code. Decoding should fail.
+TEST(GIFImageDecoderTest, badCode)
+{
+    RefPtr<SharedBuffer> testData = readFile("/Source/web/tests/data/bad-code.gif");
+    ASSERT_TRUE(testData.get());
+
+    OwnPtr<GIFImageDecoder> testDecoder(createDecoder());
+    testDecoder->setData(testData.get(), true);
+    EXPECT_EQ(1u, testDecoder->frameCount());
+    ASSERT_TRUE(testDecoder->frameBufferAtIndex(0));
+    EXPECT_TRUE(testDecoder->failed());
+}