DevTools: Do not disconnect client in InspectorFrontendHost::closeWindow

This patch removes client disconnection from
InspectorFrontendHost::closeWindow as devtools window might not be
actually closed because of beforeunload handlers.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/72603003

git-svn-id: svn://svn.chromium.org/blink/trunk@162111 bbb929c8-8fbe-4397-9dbb-9b2b20218538