Remove side-effect from RenderLayer::calculateLayerBounds

As a side effect of computing the layer bounds, this code was updating
compositing state. The state update was added in
http://src.chromium.org/viewvc/blink?view=revision&revision=162032 to fix a
chicken/egg bug. We no longer appear to need this side effect because the test
added in that CL still passes.

R=vollick@chromium.org

Review URL: https://codereview.chromium.org/226333004

git-svn-id: svn://svn.chromium.org/blink/trunk@170902 bbb929c8-8fbe-4397-9dbb-9b2b20218538