Fix chicken and egg problem in composited bounds computation

Inside RLC::rebuildCompositingTree we recursively walk the RenderLayers, and for
each layer with a backing, we first update its composited bounds, THEN we update
its geometry (naturally, since we need to know how big it is to figure out its
geometry). Problem is, we update our draws-content status when we update geometry,
and draws-content has an effect on our composited bounds. We've got a chicken
and egg problem. We must, therefore, ensure that we have updated our
draws-content status before updating our composited bounds.

Note: depends on https://codereview.chromium.org/58543002

TEST=transforms/3d/point-mapping/3d-point-mapping-deep.html
BUG=317698

Review URL: https://codereview.chromium.org/68833004

git-svn-id: svn://svn.chromium.org/blink/trunk@162032 bbb929c8-8fbe-4397-9dbb-9b2b20218538