Revert 168245 "Drop background color optimization for composited..."

Suspected of causing lots of browser_test failures on Mac:
http://build.chromium.org/p/chromium.webkit/builders/Mac10.6%20Tests/builds/23338

> 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

TBR=dongseong.hwang@intel.com

Review URL: https://codereview.chromium.org/183763024

git-svn-id: svn://svn.chromium.org/blink/trunk@168317 bbb929c8-8fbe-4397-9dbb-9b2b20218538