Delete RenderLayer::setHasVisibleContent

This function cleans out a single dirty bit in the RenderLayer tree without
cleaning the entire subtree, which breaks the invariant that dirty bits
represent are connected up to the root of the tree. Fortunately, this function
isn't needed any longer.

This CL also fixed a bug whereby we'd issue invalidations for the previous
location of an element that became visible. That can manifest as a bug if the
element moves after becoming visible or if the old rect exists in another
graphics layer.

R=esprehn@chromium.org, skobes@chromium.org

Review URL: https://codereview.chromium.org/456963002

git-svn-id: svn://svn.chromium.org/blink/trunk@180007 bbb929c8-8fbe-4397-9dbb-9b2b20218538