Set WebFont priority to very low if the network is detected to be slow

third_party/WebKit/Source/core/fetch/FontResource.h

Index: third_party/WebKit/Source/core/fetch/FontResource.h
diff --git a/third_party/WebKit/Source/core/fetch/FontResource.h b/third_party/WebKit/Source/core/fetch/FontResource.h
index e83ab315a525782642b34205c432c2d1cc84114f..e93ed7b87237ff8ddc7f43e37054bb76c4283a7e 100644
--- a/third_party/WebKit/Source/core/fetch/FontResource.h
+++ b/third_party/WebKit/Source/core/fetch/FontResource.h
@@ -67,6 +67,11 @@ class CORE_EXPORT FontResource final : public Resource {
       bool italic,
       FontOrientation = FontOrientation::Horizontal);
 
+  // Returns true if the loading priority of the remote font resource can be
+  // lowered. The loading priority of the font can be lowered only if the
+  // font is not needed for painting the text.
+  bool isLowPriorityLoadingAllowedForRemoteFont() const;
+
  private:
   class FontResourceFactory : public ResourceFactory {
    public:
@@ -112,6 +117,9 @@ class FontResourceClient : public ResourceClient {
   ResourceClientType getResourceClientType() const final { return FontType; }
   virtual void fontLoadShortLimitExceeded(FontResource*) {}
   virtual void fontLoadLongLimitExceeded(FontResource*) {}
+
+  // Returns true if loading priority of remote font resources can be lowered.
+  virtual bool isLowPriorityLoadingAllowedForRemoteFont() const { return true; }

[kinuko, 2016/11/15 05:43:47]

It probably doesn't change anything but is the default return value being true
intentional?  (I often see returning false in such cases)

[tbansal1, 2016/11/15 08:17:04]

Yes it is intentional.  I have added more comments based on offline discussion
with ksakamato and toyoshim. 

Basically, for every font resource FontResourceHelper::FontResourceHelper also
gets automatically registered as a FontResourceClient. If we set the default
return to false here, then calling this function on
FontResourceHelper::FontResourceHelper  client would return false. This will
incorrectly prevent the priority from getting lowered.

Also, since this function is only called by RemoteFontFaceSOurce, and is
pertinent only to remote fonts, it is okay for default return to be true (as
long as RemoteFontFaceSOurce correctly overrides the function).

[Kunihiko Sakamoto, 2016/11/15 09:34:53]

A supplementary explanation:
All subclasses of ResourceOwner<FontResource> are FontResourceClient, because
ResourceOwner<R> is derived from R::ClientType. So FontResourceHelper is a
FontResourceClient and can observe FontResource, but it shouldn't participate in
the decision of load priority.

 };
 
 }  // namespace blink