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-Split up the sticky and fixed position logic as they're quite
different.
-Add a CompositingTrigger for viewport constrained positioned
elements. This gets rid of the last use of settings in
CompositingReasonFinder. Each other use has been high on
profiles and it makes the code more consistent.
-Get rid of redundant checks of renderer->isPositioned(),
renderer->isOutOfFlowPositioned(),
renderer->isInFlowPositioned() and 
!layer->stackingNode()->isStackingContainer(). All those
bits are implied by the position being fixed or sticky.

Review URL: https://codereview.chromium.org/231613002

git-svn-id: svn://svn.chromium.org/blink/trunk@171504 bbb929c8-8fbe-4397-9dbb-9b2b20218538