Fix several squashing layer positioning bugs

This patch cleans up the squashingLayer positioning code a bit,
and fixes four issues: (1) The fixed-position constraint was
being applied to the squashingContainmentLayer instead of the
layer below it, (2) It's not correct to position the
squashingLayer with respect to the main GraphicsLayer of the
CompositedLayerMapping when the main GraphicsLayer is fixed
position, (3) When a layer pops out of a squashed layer, a
repaint needs to be issued correctly to the new repaint
container, and (4) the positioning of the squashingLayer was
incorrectly including some local positioning junk needed by
m_graphicsLayer that should not be included in the squashingLayer
position.

This patch is a merge of two other patches that are already
reviewed and approved:
https://codereview.chromium.org/108723005/
https://codereview.chromium.org/132363003/

TBR=vollick@chromium.org
BUG=328142

Review URL: https://codereview.chromium.org/128783006

git-svn-id: svn://svn.chromium.org/blink/trunk@164869 bbb929c8-8fbe-4397-9dbb-9b2b20218538