Fix ASSERT failure in RenderGeometryMap.

This ASSERT is too strong, and doesn't quite reflect the
intended behaviour. Invisible layers are skipped when
recomputing location in
RenderLayer::updateLayerPositionsAfterScroll(), but the
ASSERT assumes that even invisible layers should be correctly
positioned.

This patch changes the ASSERT to more accurately reflect
the correct intended behaviour, by skipping invisible
layers.

BUG=306100

Review URL: https://codereview.chromium.org/27104002

git-svn-id: svn://svn.chromium.org/blink/trunk@159922 bbb929c8-8fbe-4397-9dbb-9b2b20218538