Remove call to updateLayerCompositingState in RenderLayer::contentChanged

This call site was already trigginer a CompositingUpdateAfterCompositingInputChange.
Calling updateLayerCompositingState could upgrade to CompositingUpdateRebuildTree,
but there's no reason to do that eagerly. We compute whether that upgrade is
necessary during the compositing update itself.

BUG=343756

Review URL: https://codereview.chromium.org/327963005

git-svn-id: svn://svn.chromium.org/blink/trunk@175939 bbb929c8-8fbe-4397-9dbb-9b2b20218538