devtools: allow subscribeDebuggerEvents to be called multiple times

This fixes broken handling of reload events in intercom and broken
1.2.19 release. Also subscribeDebuggerEvents should not fail silently
but throw instead.

Author: darwin

resources/unpacked/devtools/front_end/dirac/dirac.js

@@ -164,8 +164,8 @@ Object.assign(window.dirac, (function() {
     return loadLazyDirac().then(() => window.dirac.subscribeDebuggerEvents(...args));
   }
 
-  function unsubscribeDebuggerEvents() {
-    return loadLazyDirac().then(() => window.dirac.unsubscribeDebuggerEvents());
+  function unsubscribeDebuggerEvents(...args) {
+    return loadLazyDirac().then(() => window.dirac.unsubscribeDebuggerEvents(...args));
   }
 
   function addConsoleMessageToMainTarget(...args) {

resources/unpacked/devtools/front_end/dirac_lazy/dirac_lazy.js

@@ -128,14 +128,14 @@ Object.assign(window.dirac, (function() {
     evalInContext(lookupDefaultContext(), code, silent, callback);
   }
 
-  let debuggerEventsUnsubscriber = null;
+  let debuggerEventsUnsubscribers = new Map();
 
   /**
    * @return {boolean}
    */
   function subscribeDebuggerEvents(callback) {
-    if (debuggerEventsUnsubscriber) {
-      return false;
+    if (debuggerEventsUnsubscribers.has(callback)) {
+      throw new Error("subscribeDebuggerEvents called without prior unsubscribeDebuggerEvents for callback " + callback);
     }
     const globalObjectClearedHandler = (...args) => {
       callback("GlobalObjectCleared", ...args);
@@ -151,30 +151,27 @@ Object.assign(window.dirac, (function() {
     SDK.targetManager.addModelListener(SDK.DebuggerModel, SDK.DebuggerModel.Events.DebuggerPaused, debuggerPausedHandler, window.dirac);
     SDK.targetManager.addModelListener(SDK.DebuggerModel, SDK.DebuggerModel.Events.DebuggerResumed, debuggerResumedHandler, window.dirac);
 
-    /**
-     * @return {boolean}
-     */
-    debuggerEventsUnsubscriber = () => {
+    debuggerEventsUnsubscribers.set(callback, () => {
       SDK.targetManager.removeModelListener(SDK.DebuggerModel, SDK.DebuggerModel.Events.GlobalObjectCleared, globalObjectClearedHandler, window.dirac);
       SDK.targetManager.removeModelListener(SDK.DebuggerModel, SDK.DebuggerModel.Events.DebuggerPaused, debuggerPausedHandler, window.dirac);
       SDK.targetManager.removeModelListener(SDK.DebuggerModel, SDK.DebuggerModel.Events.DebuggerResumed, debuggerResumedHandler, window.dirac);
       return true;
-    };
+    });
 
     return true;
   }
 
   /**
    * @return {boolean}
    */
-  function unsubscribeDebuggerEvents() {
-    if (!debuggerEventsUnsubscriber) {
-      return false;
+  function unsubscribeDebuggerEvents(callback) {
+    if (!debuggerEventsUnsubscribers.has(callback)) {
+      throw new Error("unsubscribeDebuggerEvents called without prior subscribeDebuggerEvents for callback " + callback);
     }
 
-    const res = debuggerEventsUnsubscriber();
-    debuggerEventsUnsubscriber = null;
-    return res;
+    const unsubscriber = debuggerEventsUnsubscribers.get(callback);
+    debuggerEventsUnsubscribers.delete(callback);
+    return unsubscriber();
   }
 
   // --- console ----------------------------------------------------------------------------------------------------------

resources/unpacked/devtools/front_end/externs.js

@@ -438,9 +438,10 @@ var dirac = {
      */
     subscribeDebuggerEvents: function(callback) {},
     /**
+     * @param {?} callback
      * @return {boolean}
      */
-    unsubscribeDebuggerEvents: function() {},
+    unsubscribeDebuggerEvents: function(callback) {},
 
     /**
     * @param {?} callFrame