Remove 'hostId()' from WebDevToolsAgentImpl.h

Minor cleanup. It's never called anywhere.

BUG=none.

Review URL: https://codereview.chromium.org/99963006

git-svn-id: svn://svn.chromium.org/blink/trunk@163142 bbb929c8-8fbe-4397-9dbb-9b2b20218538