When we change our compositing state, reflection follows

Previously we were calling reflectionLayer->setCompositingReasons(reflectionLayer->compositingReasons() |reflectionCompositingReason). This meant if CompositingReasonReflectionOfCompositedParent was already set it would remain set because we are simply or-ing. This updates the code to mask off that single bit and set/unset it.

BUG=411855

Review URL: https://codereview.chromium.org/581233002

git-svn-id: svn://svn.chromium.org/blink/trunk@182335 bbb929c8-8fbe-4397-9dbb-9b2b20218538