Remove some unnecessary settings null-checks from RenderLayerCompositor.

We only call these methods in the middle of layout or compositing update.
settings() only returns null on detached documents. We shouldn't
ever call layout or compositing update on a detached document.

Tested locally with an assert that we never have a null settings.
I don't think the assert is actually necessary to check in though.
If we started hitting this code with a null settings we'd just
have a crash at the next line of code anyways.

Also, cleaned up RenderLayerCompositor::updateAcceleratedCompositingSettings
a bit to not use local variables as much. This does mean we
call updateTriggers even if accelerated compositing is not enabled,
but that seems fine.

Review URL: https://codereview.chromium.org/239723003

git-svn-id: svn://svn.chromium.org/blink/trunk@171714 bbb929c8-8fbe-4397-9dbb-9b2b20218538