Fix case mismatch between blink::ImageDecoder and MimeUtil.

MimeUtil::IsSupportedImageMimeType() always lowers case before testing
for support, however ImageDecoder::CreateByMimeType() leaves the case
untouched. This change forces lower case in WebCodecs ImageDecoder and
ImageDecoder::CreateByMimeType to ensure consistency.

R=pkasting

Fixed: 1168980
Change-Id: Ibe4202fb56f7b1a0731553662607c76e207467cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2643270
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845877}

third_party/blink/renderer/modules/webcodecs/image_decoder_external.cc

@@ -73,7 +73,7 @@ ImageDecoderExternal::ImageDecoderExternal(ScriptState* script_state,
   options_ =
       init->hasOptions() ? init->options() : ImageBitmapOptions::Create();
 
-  mime_type_ = init->type();
+  mime_type_ = init->type().LowerASCII();
   if (!canDecodeType(mime_type_)) {
     exception_state.ThrowDOMException(DOMExceptionCode::kNotSupportedError,
                                       "Unsupported image format");
@@ -311,7 +311,7 @@ void ImageDecoderExternal::CreateImageDecoder() {
 
   // CreateByImageType() can't fail if we use a supported image type. Which we
   // DCHECK above via canDecodeType().
-  DCHECK(decoder_);
+  DCHECK(decoder_) << mime_type_;
 }
 
 void ImageDecoderExternal::MaybeSatisfyPendingDecodes() {

third_party/blink/renderer/modules/webcodecs/image_decoder_external_test.cc

@@ -154,6 +154,17 @@ TEST_F(ImageDecoderTest, DecodeUnsupported) {
   EXPECT_TRUE(v8_scope.GetExceptionState().HadException());
 }
 
+TEST_F(ImageDecoderTest, DecoderCreationMixedCaseMimeType) {
+  V8TestingScope v8_scope;
+  constexpr char kImageType[] = "image/GiF";
+  EXPECT_TRUE(ImageDecoderExternal::canDecodeType(kImageType));
+  auto* decoder =
+      CreateDecoder(&v8_scope, "images/resources/animated.gif", kImageType);
+  ASSERT_TRUE(decoder);
+  ASSERT_FALSE(v8_scope.GetExceptionState().HadException());
+  EXPECT_EQ(decoder->type(), "image/gif");
+}
+
 TEST_F(ImageDecoderTest, DecodeGif) {
   V8TestingScope v8_scope;
   constexpr char kImageType[] = "image/gif";

third_party/blink/renderer/platform/image-decoders/image_decoder.cc

@@ -181,8 +181,9 @@ std::unique_ptr<ImageDecoder> ImageDecoder::CreateByMimeType(
       CalculateMaxDecodedBytes(high_bit_depth_decoding_option, desired_size);
 
   // Note: The mime types below should match those supported by
-  // MimeUtil::IsSupportedImageMimeType().
+  // MimeUtil::IsSupportedImageMimeType() (which forces lowercase).
   std::unique_ptr<ImageDecoder> decoder;
+  mime_type = mime_type.LowerASCII();
   if (mime_type == "image/jpeg" || mime_type == "image/pjpeg" ||
       mime_type == "image/jpg") {
     decoder = std::make_unique<JPEGImageDecoder>(alpha_option, color_behavior,