Looks like manual vsync ticks are not generated when scheduler is updating resources. This happens when there's nothing to draw but resources to update. To reproduce go to a page that generates more than 48 resource updates per frame. E.g. http://www.corp.google.com/~reveman/texture-uploads/test.html with the browser window relatively large. Or it will reproduce on any composited page after lowering the textureUpdatesPerFrame constant in CCTextureUpdateController.cpp to 5.
I think Brian just fixed this today, or has a fix in progress: https://bugs.webkit.org/show_bug.cgi?id=94012
yup, that's the same issue. thanks.
*** This bug has been marked as a duplicate of bug 94012 ***