Painters should not modify LayoutObject status except some status
dedicated for painting. The exceptions are marked mutable.

List of 'mutable's added in this CL:
- FrameView::m_isPainting: for assertion (in FrameView::layout(),
  Document::updateLayoutTrees() and FramePainter::paintContents()).
  Will change to document cycle states.
- LayoutObject::BitFields::m_boxDecorationBackgroundState: Lazily
  updated in LayoutObject::boxDecorationBackgroundIsKnownToBeObscured().
- LayoutTable::m_currentBorder: Used temporarily during painting only.
  Will move into painter.

List of 'const_cast's added in this CL:
- ImageQualityController::highQualityRepaintTimerFired()
  when invalidating LayoutObject on timer. Will change to
  mutableForPainting[1].
- LineBoxListPainter::paint()
  This is of const sementics, but LineLayoutBoxModel doesn't support
  const for now (crbug.com/535785)
- SVGFilterPainter::prepareEffect()
  when calling m_filter.setFilterDataForLayoutObject. TODO(pdr).
- ScrollbarPainter::paintIntoRect()
  a layer violation that the painter changes geometry of scrollbars.
  There is an existing FIXME.
- When calling get() on a Map<non-const-pointer, ...>.
- Used to implement const version of getters based on the non-const
  version.

[1] mutableForPainting: https://codereview.chromium.org/1363613002/#msg6

Review URL: https://codereview.chromium.org/1361363003

Cr-Commit-Position: refs/heads/master@{#351868}