[DevTools] Remove InspectorBackend.registerXXXXDispatcher methods.

We should register dispatchers on the target.

BUG=none

Review URL: https://codereview.chromium.org/605723002

git-svn-id: svn://svn.chromium.org/blink/trunk@182691 bbb929c8-8fbe-4397-9dbb-9b2b20218538