Do not squash invisible layers.

We used to squash RenderLayers even if they reported that
subtreeIsInvisible. This caused the creation of needlessly enormous
layers that contained nothing. This CL excludes those layers from
the list of squashed layers.

R=abarth@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/212923005

git-svn-id: svn://svn.chromium.org/blink/trunk@170250 bbb929c8-8fbe-4397-9dbb-9b2b20218538