DevTools: pass agent host id to the renderer (Blink).

public/web/WebDevToolsAgentClient.h

Index: public/web/WebDevToolsAgentClient.h
diff --git a/public/web/WebDevToolsAgentClient.h b/public/web/WebDevToolsAgentClient.h
index ea514f42ca36b47e82b9c4135f38a2c713f0ac45..23730ee6749dbd3f1db827ae5e5f13d6deb728a0 100644
--- a/public/web/WebDevToolsAgentClient.h
+++ b/public/web/WebDevToolsAgentClient.h
@@ -47,8 +47,12 @@ public:
     virtual void sendDebuggerOutput(const WebString&) { }
 
     // Returns the identifier of the entity hosting this agent.
+    // FIXME: remove once migrated to debuggerId().
     virtual int hostIdentifier() { return -1; }
 
+    // Returns unique identifier of the entity within process.
+    virtual int debuggerId() { return hostIdentifier(); }
+
     // Save the agent state in order to pass it later into WebDevToolsAgent::reattach
     // if the same client is reattached to another agent.
     virtual void saveAgentRuntimeState(const WebString&) { }