Add RenderLayerCompositor::m_rootTransformLayer

If we apply the transform at a different level in the tree, it no
no longer affects the HUD.

BUG=344295, 299837

Review URL: https://codereview.chromium.org/170153002

git-svn-id: svn://svn.chromium.org/blink/trunk@167579 bbb929c8-8fbe-4397-9dbb-9b2b20218538