Refactor ResourceLoadPriorityOptimizer to avoid walking render tree after every layout and scroll event.

1. Instead of calling RenderObject::didScroll on every
FrameView::scrollPositionChanged, use a 250 ms timer to update the
image priorities. Ideally, we only update the image priorities once
now about 250 ms after scrolling stops.

2. Whenever a RenderImage/RenderBlock is created, it's added to the
ResourceLoadPriorityOptimizer. After a layout/scroll, iterate the list
removing any RenderObject that either has no images, or has loaded all
of it's images.


BUG=332285

Review URL: https://codereview.chromium.org/131233003

git-svn-id: svn://svn.chromium.org/blink/trunk@165582 bbb929c8-8fbe-4397-9dbb-9b2b20218538