Avoid (theorethical) NPE.

All forwarded methods, expect didReceiveResponse, contains m_client check.

Added check to didReceiveResponse for consistency.

BUG=356654

Review URL: https://codereview.chromium.org/213383008

git-svn-id: svn://svn.chromium.org/blink/trunk@170279 bbb929c8-8fbe-4397-9dbb-9b2b20218538