Removed most calls to GraphicsContext3D from DrawingBuffer

Also includes some misc related refactors that were missed on previous CLs

BUG=326382

Review URL: https://codereview.chromium.org/127493002

git-svn-id: svn://svn.chromium.org/blink/trunk@165020 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Changing from opacity less than 1 to opacity 1 should not trigger layout

RenderStyle::diff would always return StyleDifferenceLayout when adding
or removing an opacity even though layout is only required if doing so
would change if the layer is a stacking context.

See:
http://www.w3.org/TR/css3-color/#transparency
http://www.w3.org/TR/CSS21/zindex.html

BUG=334033

Review URL: https://codereview.chromium.org/136903004

git-svn-id: svn://svn.chromium.org/blink/trunk@165021 bbb929c8-8fbe-4397-9dbb-9b2b20218538