RenderLayer::hasVisibleContent() incorrect when layer removed

Prior to this CL, we were incorrectlyd dirtying
RenderLayer::hasVisibleContent() when removing a RenderLayer from the layer
tree. This caused us to incorrectly believe that the parent layer lacked
visible content.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/185653011

git-svn-id: svn://svn.chromium.org/blink/trunk@168359 bbb929c8-8fbe-4397-9dbb-9b2b20218538