Fix CanDecodeType test failure if AVIF is disabled

Fix the ImageDecoderTest.CanDecodeType test failure when the AVIF
feature is disabled.

The ImageDecoderExternal::canDecodeType() method ultimately depends on
what is added to image_types_ in the MimeUtil constructor:

  MimeUtil::MimeUtil() {
    ...
    for (const char* type : kSupportedImageTypes)
      image_types_.insert(type);
  #if BUILDFLAG(ENABLE_AV1_DECODER)
    ...
    if (base::FeatureList::IsEnabled(features::kAVIF))
      image_types_.insert("image/avif");
  #endif

So the return value of ImageDecoderExternal::canDecodeType("image/avif")
should match base::FeatureList::IsEnabled(features::kAVIF).

Test:
blink_unittests --enable-features=AVIF \
    --gtest_filter=ImageDecoderTest.CanDecodeType
blink_unittests --disable-features=AVIF \
    --gtest_filter=ImageDecoderTest.CanDecodeType

Bug: 1116502
Change-Id: Idb5a13f5cc4e78c0d9671b7460601a412936bec9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357961Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Wan-Teh Chang <wtc@google.com>
Cr-Commit-Position: refs/heads/master@{#798361}

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

@@ -4,8 +4,10 @@
 
 #include "third_party/blink/renderer/modules/webcodecs/image_decoder_external.h"
 
+#include "base/feature_list.h"
 #include "media/media_buildflags.h"
 #include "testing/gtest/include/gtest/gtest.h"
+#include "third_party/blink/public/common/features.h"
 #include "third_party/blink/renderer/bindings/core/v8/script_promise_tester.h"
 #include "third_party/blink/renderer/bindings/core/v8/v8_binding_for_testing.h"
 #include "third_party/blink/renderer/bindings/modules/v8/v8_image_decoder_init.h"
@@ -71,7 +73,8 @@ TEST_F(ImageDecoderTest, CanDecodeType) {
   EXPECT_TRUE(ImageDecoderExternal::canDecodeType("image/x-xbitmap"));
 
 #if BUILDFLAG(ENABLE_AV1_DECODER)
-  EXPECT_TRUE(ImageDecoderExternal::canDecodeType("image/avif"));
+  EXPECT_EQ(ImageDecoderExternal::canDecodeType("image/avif"),
+            base::FeatureList::IsEnabled(features::kAVIF));
 #else
   EXPECT_FALSE(ImageDecoderExternal::canDecodeType("image/avif"));
 #endif