Simplify InlineFlowBox::paintBoxDecorations.

This change:
* simplifies the paintPhase checks in InlineFlowBox::paint
* clarifies and moves the "should paint box decorations" check above
  other work, including the roundedFrameRect computation
* makes some internal painting methods private
* fixes the name of "adjustedPaintOffset"
* resolves lint errors that arose during the above

3% reduction in record time on desktop Wikipedia.
Negligible impact on N7 key_silk_cases.

BUG=388946

Review URL: https://codereview.chromium.org/352903002

git-svn-id: svn://svn.chromium.org/blink/trunk@177180 bbb929c8-8fbe-4397-9dbb-9b2b20218538