Revert 163110 "This CL attempts to improve upon the default load..."

> This CL attempts to improve upon the default loading priorities for images. By default, images will start at ResourceLoadPriorityVeryLow and after a layout or during a scroll, anything that's visible will be promoted up to ResourceLoadPriorityLow.
> 
> In our tests, this was a really nice win on speed-index, with a gain of ~5% on average, at ~14% at the 99th percentile.
> 
> BUG=319073
> 
> Review URL: https://codereview.chromium.org/86533002

TBR=simonhatch@chromium.org

Review URL: https://codereview.chromium.org/108283004

git-svn-id: svn://svn.chromium.org/blink/trunk@163362 bbb929c8-8fbe-4397-9dbb-9b2b20218538