RenderLayer::hasVisibleContent should ASSERT(!m_visibleContentStatusDirty)

This CL makes RenderLayer::hasVisibleContent ASSERT that the state it reads
isn't dirty. This CL also fixes a couple callers that were reading the state
while it was dirty.

R=skobes@chromium.org

Review URL: https://codereview.chromium.org/453113002

git-svn-id: svn://svn.chromium.org/blink/trunk@179990 bbb929c8-8fbe-4397-9dbb-9b2b20218538