Fix rounding issues in the child transform layer

We want the child transform layer to be positioned relative 
to the compositing ancestor. The child transform layer is a 
descendant of m_graphicsLayer, and applying
contentOffsetInCompositingLayer *almost undoes m_graphicsLayer's
position and gets us in the right spot, but it doesn't take
into account any of the rounding that goes into m_graphicsLayer's
position. In this patch set, the position of the child
containment layer is derived directly from m_graphicsLayer's
position to ensure consistent rounding.

BUG=402505

Review URL: https://codereview.chromium.org/461323002

git-svn-id: svn://svn.chromium.org/blink/trunk@181332 bbb929c8-8fbe-4397-9dbb-9b2b20218538