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

git-svn-id: svn://svn.chromium.org/blink/trunk@163110 bbb929c8-8fbe-4397-9dbb-9b2b20218538