New UMA stats for stacking container opt-in measurements.

Along with https://codereview.chromium.org/23874003/, this
adds Renderer.CompositedScrolling UMA histogram.

This records the total number of RenderLayers that are scrollable
areas, need to become stacking containers, and will use composited
scrolling.

Each of the buckets should be recorded at most once per
RenderLayer. The ratio between these stats should give
us an idea of how effective stacking containers are at
allowing scrollable divs to become composited.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23455031

git-svn-id: svn://svn.chromium.org/blink/trunk@157227 bbb929c8-8fbe-4397-9dbb-9b2b20218538