Improve GIF decoding performance

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

Index: Source/core/platform/image-decoders/gif/GIFImageDecoderTest.cpp
diff --git a/Source/core/platform/image-decoders/gif/GIFImageDecoderTest.cpp b/Source/core/platform/image-decoders/gif/GIFImageDecoderTest.cpp
index 090e0d90e3062264bce92ed3317f4b03e6a7a2f8..e505e9dfe49f2311a1f5e608a51a7d63f29fc957 100644
--- a/Source/core/platform/image-decoders/gif/GIFImageDecoderTest.cpp
+++ b/Source/core/platform/image-decoders/gif/GIFImageDecoderTest.cpp
@@ -451,3 +451,29 @@ TEST(GIFImageDecoderTest, resumePartialDecodeAfterClearFrameBufferCache)
     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());
+}