Allow WebFrame creators to pass in a null client.

As it turns out, some embedders simply don't care about the
WebFrameClient callbacks, so forcing them to pass in an empty one is
counterproductive.

BUG=331580

Review URL: https://codereview.chromium.org/106363008

git-svn-id: svn://svn.chromium.org/blink/trunk@164476 bbb929c8-8fbe-4397-9dbb-9b2b20218538