Fix scroll-compensation for fixed position layers within fixed position layers

When there is a fixed position layer that is a descendant of another
fixed position layer, but both of them are fixed to the same container,
we actually only need to apply scroll compensation to the first fixed
position layer (the ancestor one). The descendant fixed position layer
will remain fixed by virtue of being positioned with respect to the
ancestor fixed position layer.

On the other hand, if the descendant fixed position layer is fixed
to a different container, it needs its own scroll compensation again.

BUG=279033
NOTRY=true

Review URL: https://codereview.chromium.org/25425002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226690 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226690