Fix crash with composited layers - DO NOT MERGE

Fix crash with composited layers - DO NOT MERGE

Cherry-pick from master

When we have composited layers inside iframes/frames, the layers
hierarchy is not always up to date at the time of the layerSync()
call. If some of those layers are scheduled to be repainted, the
repaint operation will triggers the update of the composited layers
tree -- possibly resulting in the deallocation of the very same
GraphicsLayer we were painting from, and thus leading to a crash.

The fix consist in gathering all the root RenderLayer (for each
frame containing composited layers) and explicitely asking
RenderLayerCompositor to check if the composited tree needs to be
updated, before we traverse the tree to paint the elements.

bug:5695185
Change-Id: I33a00b847eb19c9aa4b68f0ac3adbe36709ed00b