Update scrollbar layer opacity when scrollbar theme changes

If the scrollbar theme changes from being opaque to non-opaque (as happen
when unplugging an external mouse on Mac), a situation results where the
scrollbar layers think they were opaque (because they were when they were
created), but the content that they draw assumes that it is not opaque, and
black scrollbars result.

BUG=383205

Review URL: https://codereview.chromium.org/383603002

git-svn-id: svn://svn.chromium.org/blink/trunk@178421 bbb929c8-8fbe-4397-9dbb-9b2b20218538