DevTools: untangle paint profiler for better reuse

Make PaintProfilerView and PaintProfilerCommandLogView independent on
each other, make them both children of LayerPaintProfilerView and remove
layer-specific stuff into the latter, so that the former two operate on
just snapshots.

BUG=

Review URL: https://codereview.chromium.org/340043002

git-svn-id: svn://svn.chromium.org/blink/trunk@176454 bbb929c8-8fbe-4397-9dbb-9b2b20218538