Delete CompositingUpdateAfterLayout

Rather than using a different update type, we can just mark the root as needing
its properties updated, which will cause us to update the entire tree. This CL
alone isn't a huge win, but it will let us remove
RenderLayerCompositor::m_compositingLayersNeedRebuild in the next CL.

Also, move the call to enableCompositingModeIfNeeded into the new
RenderLayerCompositor::didLayout function. This makes the sequence look a bit
silly (because we mark the dirty bit only to clear it immediately), but that's
what the code did before (albeit in a more roundabout way).

BUG=332248

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175738

Review URL: https://codereview.chromium.org/323653002

git-svn-id: svn://svn.chromium.org/blink/trunk@175752 bbb929c8-8fbe-4397-9dbb-9b2b20218538