Always handle "avif" in FileExtensionToImageType()

Always handle "avif" in FileExtensionToImageType(), whether the "AVIF"
feature is enabled or not.

The file-scope internal function FileExtensionToImageType() is only
called on the return value of the virtual method
ImageDecoder::FilenameExtension(). Therefore, the only way the
image_extension argument is equal to "avif" is that
AVIFImageDecoder::FilenameExtension() is called. So it does not need to
check if the AVIF feature is enabled, because we still want to run
AVIFImageDecoder unit tests when the AVIF feature is disabled.

Test:
blink_platform_unittests --enable-features=AVIF --gtest_filter=*AVIF*
blink_platform_unittests --disable-features=AVIF --gtest_filter=*AVIF*

Bug: 1116502
Change-Id: I17e0290fef25a508c4e87cbdc07a27ef0dac73b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357957Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Wan-Teh Chang <wtc@google.com>
Cr-Commit-Position: refs/heads/master@{#798384}

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

@@ -61,10 +61,8 @@ cc::ImageType FileExtensionToImageType(String image_extension) {
   if (image_extension == "bmp")
     return cc::ImageType::kBMP;
 #if BUILDFLAG(ENABLE_AV1_DECODER)
-  if (base::FeatureList::IsEnabled(features::kAVIF) &&
-      image_extension == "avif") {
+  if (image_extension == "avif")
     return cc::ImageType::kAVIF;
-  }
 #endif
   return cc::ImageType::kInvalid;
 }