Speculative fix for crash in WebPagePopupImpl::initialize.

ClusterFuzz says there is a null pointer dereference in the function. I suspect
m_widgetClient can be null.

BUG=383150

Review URL: https://codereview.chromium.org/365503006

git-svn-id: svn://svn.chromium.org/blink/trunk@177263 bbb929c8-8fbe-4397-9dbb-9b2b20218538