Invalidate cached clip rects in all descendants of a layer

When clearing cached clip rects from a layer and its descendants do not
return early when we encounter a layer that has no cached clip rects.
There's nothing to ensure that descendants further down the tree do not
have cached rects that need to be cleared.

In the page that revealed this bug (https://imgur.com/a/8ESMW and any
other imgur page that has an album panel) removing the early return
increases the calls to the clearClipRectsIncludingDescendants (c. 400 ->
600) when you hover over the album panel. The numbers from general
scrolling around and loading don't increase too much (with and without
the early return there are about 5000 calls in total when you load the 
page and scroll all the way down).

With the early return this function is called a couple of thousand times
when loading guardian.co.uk and up to 4 or 5 thousand times when mousing
across content that animates when hovered over. Without the early return
these numbers seem pretty much the same.

So it looks like this function is not especially hot and adding a book-keeping
mechanism will probably create as much work as it will save.

BUG=405683

Review URL: https://codereview.chromium.org/468803005

git-svn-id: svn://svn.chromium.org/blink/trunk@181058 bbb929c8-8fbe-4397-9dbb-9b2b20218538