WebViewImpl::scheduleAnimation should null-check its m_client poitner

m_client is null in the SharedWorker process, we need to always null-check.

I found this by attempting to move layout scheduling to the compositor
and caused all SharedWorker tests to crash.

R=jamesr@chromium.org
BUG=264489

Review URL: https://codereview.chromium.org/20555004

git-svn-id: svn://svn.chromium.org/blink/trunk@155025 bbb929c8-8fbe-4397-9dbb-9b2b20218538