Fix image preloading for <picture> elements with explicitly empty image types

HTMLPreloadScanner and HTMLImageElement were using slightly different
logic to determine if an image type is supported. In particular,
HTMLImageElement considered an empty image type as supported and
HTMLPreloadScanner did not. In the following case, it'd result
in HTMLPreloadScanner trying to preload a resource (.png) that will not
be used by HTMLImageElement (it will use .avif):

```
<picture>
  <source srcset="images/test.avif" type="">
  <img src="images/test.png">
</picture>
```

This CL adds a static method HTMLImageElement::SupportedImageType
that is used by HTMLPreloadScanner to check if an image type is
supported. Also, redundant checks for type emptiness are removed
from HTMLImageElement.

Fixed: 1141776
Change-Id: I89ed95c18bbbc86613f6b362ba935f428e1d05b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2494720
Commit-Queue: Alex Rudenko <alexrudenko@chromium.org>
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#820206}

third_party/blink/renderer/core/html/html_image_element.cc

@@ -335,7 +335,7 @@ String HTMLImageElement::AltText() const {
   return FastGetAttribute(html_names::kTitleAttr);
 }
 
-static bool SupportedImageType(const String& type) {
+bool HTMLImageElement::SupportedImageType(const String& type) {
   String trimmed_type = ContentType(type).GetType();
   // An empty type attribute is implicitly supported.
   if (trimmed_type.IsEmpty())
@@ -367,7 +367,7 @@ ImageCandidate HTMLImageElement::FindBestFitImageFromPictureParent() {
     if (srcset.IsEmpty())
       continue;
     String type = source->FastGetAttribute(html_names::kTypeAttr);
-    if (!type.IsEmpty() && !SupportedImageType(type))
+    if (!SupportedImageType(type))
       continue;
 
     if (!source->MediaQueryMatches())

third_party/blink/renderer/core/html/html_image_element.h

@@ -181,6 +181,8 @@ class CORE_EXPORT HTMLImageElement final
   void SetIsAdRelated() { is_ad_related_ = true; }
   bool IsAdRelated() const override { return is_ad_related_; }
 
+  static bool SupportedImageType(const String& type);
+
  protected:
   // Controls how an image element appears in the layout. See:
   // https://html.spec.whatwg.org/C/#image-request

third_party/blink/renderer/core/html/parser/html_preload_scanner.cc

@@ -499,8 +499,7 @@ class TokenPreloadScanner::StartTagScanner {
       // FIXME - Don't match media multiple times.
       matched_ &= MediaAttributeMatches(*media_values_, attribute_value);
     } else if (Match(attribute_name, html_names::kTypeAttr)) {
-      matched_ &= MIMETypeRegistry::IsSupportedImagePrefixedMIMEType(
-          ContentType(attribute_value).GetType());
+      matched_ &= HTMLImageElement::SupportedImageType(attribute_value);
     }
   }
 
@@ -633,14 +632,13 @@ class TokenPreloadScanner::StartTagScanner {
              MIMETypeRegistry::IsSupportedStyleSheetMIMEType(
                  ContentType(type_attribute_value_).GetType());
     } else if (link_is_preload_) {
+      if (type == ResourceType::kImage)
+        return HTMLImageElement::SupportedImageType(type_attribute_value_);
       if (type_attribute_value_.IsEmpty())
         return true;
       String type_from_attribute = ContentType(type_attribute_value_).GetType();
       if ((type == ResourceType::kFont &&
            !MIMETypeRegistry::IsSupportedFontMIMEType(type_from_attribute)) ||
-          (type == ResourceType::kImage &&
-           !MIMETypeRegistry::IsSupportedImagePrefixedMIMEType(
-               type_from_attribute)) ||
           (type == ResourceType::kCSSStyleSheet &&
            !MIMETypeRegistry::IsSupportedStyleSheetMIMEType(
                type_from_attribute))) {

third_party/blink/renderer/core/html/parser/html_preload_scanner_test.cc

@@ -761,6 +761,10 @@ TEST_F(HTMLPreloadScannerTest, testPicture) {
       {"http://example.test",
        "<picture><source srcset='srcset_bla.gif'><img src='bla.gif'></picture>",
        "srcset_bla.gif", "http://example.test/", ResourceType::kImage, 0},
+      {"http://example.test",
+       "<picture><source srcset='srcset_bla.gif' type=''><img "
+       "src='bla.gif'></picture>",
+       "srcset_bla.gif", "http://example.test/", ResourceType::kImage, 0},
       {"http://example.test",
        "<picture><source sizes='50vw' srcset='srcset_bla.gif'><img "
        "src='bla.gif'></picture>",