Merge updateVisibilityAfterStyleChange into RenderStyle::styleChanged

This function early outs, but it wasn't clear that it was doing that from
inside RenderLayer::styleChanged. Lets merge it back in since it's only
one line anyway and also add an ASSERT that the diff is >= repaint if the
visibility changed.

BUG=335703

Review URL: https://codereview.chromium.org/148963002

git-svn-id: svn://svn.chromium.org/blink/trunk@165963 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Teach garden-o-matic not to step on its own tail.

Because googlestorage may take non-trivially long time to fetch
results, garden-o-matic may occasionally initiate an update
cycle before the previous one completed, creating all kinds
of fun effects.

This is the simplest fix for now, just to stop an update cycle
when one is already in progress.

R=abarth@chromium.org, abarth, dpranke
BUG=307201

Review URL: https://codereview.chromium.org/149083002

git-svn-id: svn://svn.chromium.org/blink/trunk@165964 bbb929c8-8fbe-4397-9dbb-9b2b20218538