Only call updateReflectionInfo in RenderLayer::styleChanged when reflection did change

It doesn't seem we had any early outs for reflections which is nasty. It
looks like once you added a reflection we'd keep mallocing up new transforms
inside RenderLayerReflectionInfo::updateAfterStyleChange on every style
recalc even if the reflection didn't change.

This patch fixes that and adds asserts.

BUG=335703

Review URL: https://codereview.chromium.org/139093008

git-svn-id: svn://svn.chromium.org/blink/trunk@166003 bbb929c8-8fbe-4397-9dbb-9b2b20218538