Update mojo's WebMimeRegistry for supportsImagePrefixed

This is a new pure virtual on WebMimeRegistry.

The implementations of WebMimeRegistry appear to bind only to net, so
they could go somewhere and be shared by both content/ and mojo/ instead
of duplicated. But this fixes a pending compile error so should probably
happen first.

R=darin@chromium.org

Review URL: https://codereview.chromium.org/511583002

Cr-Commit-Position: refs/heads/master@{#291981}

mojo/services/html_viewer/webmimeregistry_impl.cc

@@ -35,6 +35,17 @@ blink::WebMimeRegistry::SupportsType WebMimeRegistryImpl::supportsImageMIMEType(
       blink::WebMimeRegistry::IsNotSupported;
 }
 
+blink::WebMimeRegistry::SupportsType
+WebMimeRegistryImpl::supportsImagePrefixedMIMEType(
+    const blink::WebString& mime_type) {
+  std::string ascii_mime_type = ToASCIIOrEmpty(mime_type);
+  return (net::IsSupportedImageMimeType(ascii_mime_type) ||
+          (StartsWithASCII(ascii_mime_type, "image/", true) &&
+           net::IsSupportedNonImageMimeType(ascii_mime_type)))
+             ? WebMimeRegistry::IsSupported
+             : WebMimeRegistry::IsNotSupported;
+}
+
 blink::WebMimeRegistry::SupportsType
     WebMimeRegistryImpl::supportsJavaScriptMIMEType(
     const blink::WebString& mime_type) {

mojo/services/html_viewer/webmimeregistry_impl.h

@@ -20,6 +20,8 @@ class WebMimeRegistryImpl : public blink::WebMimeRegistry {
       const blink::WebString& mime_type);
   virtual blink::WebMimeRegistry::SupportsType supportsImageMIMEType(
       const blink::WebString& mime_type);
+  virtual blink::WebMimeRegistry::SupportsType supportsImagePrefixedMIMEType(
+      const blink::WebString& mime_type);
   virtual blink::WebMimeRegistry::SupportsType supportsJavaScriptMIMEType(
       const blink::WebString& mime_type);
   virtual blink::WebMimeRegistry::SupportsType supportsMediaMIMEType(