Chrome::client() should return a ChromeClient reference.

It is never null. We can remove some unnecessary null checks.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/22955006

git-svn-id: svn://svn.chromium.org/blink/trunk@156375 bbb929c8-8fbe-4397-9dbb-9b2b20218538