Drop background color optimization for composited layers

Rationale
1. GraphicsLayer deals with background-color layer as a contents layer, so a
background-color layer is in trouble in terms of the correct order with negative
z-order children.
2. CompositedLayerMapping has the big hack to decide the given layer can be
background-color layer. Unfortunately, the big hack is incomplete and invasive.
3. Hybrid Accelerated Rasterization will cover this optimization in more generic way.
4. background-color layer covers very limited cases. A layer-promoted box
object must not have children, not have decoration, only have a background-color
property.

BUG=114658, 340691
TEST=compositing/background-color/background-color-drawn-over-child.html,
compositing/*

Review URL: https://codereview.chromium.org/178013003

git-svn-id: svn://svn.chromium.org/blink/trunk@168245 bbb929c8-8fbe-4397-9dbb-9b2b20218538