Add an ASSERT that we paint a laid-out tree

This change adds an ASSERT to the 2 entry points into painting:
- FrameView::paintContent
- RenderLayerBacking::doPaintTask

This is less fragile and intrusive than adding the ASSERT to all
the painting functions.

This change also consolidates the assertion code around 2 methods
in RenderObject that are reused throughout the code.

Review URL: https://chromiumcodereview.appspot.com/23431024

git-svn-id: svn://svn.chromium.org/blink/trunk@157732 bbb929c8-8fbe-4397-9dbb-9b2b20218538