Foreground layer position must depend on child transform layer position.

The foreground layer is currently ignorant of the child transform layer
which causes difficulties when the child transform layer is positioned.
In that case the foreground contents are offset by the child transform
layer's position. This CL adds logic to the m_foregroundLayer geometry
computation to ensure that its position is no longer incorrectly
affected by the child transform layer's position.

BUG=372183

Review URL: https://codereview.chromium.org/288543002

git-svn-id: svn://svn.chromium.org/blink/trunk@173963 bbb929c8-8fbe-4397-9dbb-9b2b20218538