refactor: consolidate all events in Events.js (#3772)

This will allow us to break all dependency cycles that were forcing
us to put many things in a single file (e.g. ExecutionContext and
ElementHandle).

lib/Connection.js

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 const {helper, assert} = require('./helper');
+const {Events} = require('./Events');
 const debugProtocol = require('debug')('puppeteer:protocol');
 const debugSession = require('debug')('puppeteer:session');
 const EventEmitter = require('events');
@@ -120,7 +121,7 @@ class Connection extends EventEmitter {
     for (const session of this._sessions.values())
       session._onClosed();
     this._sessions.clear();
-    this.emit(Connection.Events.Disconnected);
+    this.emit(Events.Connection.Disconnected);
   }
 
   dispose() {
@@ -140,10 +141,6 @@ class Connection extends EventEmitter {
   }
 }
 
-Connection.Events = {
-  Disconnected: Symbol('Connection.Events.Disconnected'),
-};
-
 class CDPSession extends EventEmitter {
   /**
    * @param {!Connection|!CDPSession} connection
@@ -228,7 +225,7 @@ class CDPSession extends EventEmitter {
       callback.reject(rewriteError(callback.error, `Protocol error (${callback.method}): Target closed.`));
     this._callbacks.clear();
     this._connection = null;
-    this.emit(CDPSession.Events.Disconnected);
+    this.emit(Events.CDPSession.Disconnected);
   }
 
   /**
@@ -242,10 +239,6 @@ class CDPSession extends EventEmitter {
   }
 }
 
-CDPSession.Events = {
-  Disconnected: Symbol('CDPSession.Events.Disconnected'),
-};
-
 helper.tracePublicAPI(CDPSession);
 
 /**