feat(ScreenObtainer): CHROME_EXTENSION_USER_GESTURE_REQUIRED

CHROME_EXTENSION_USER_GESTURE_REQUIRED will be triggered if
the extension installation fails, because the request to get desktop
stream did not originate in user gesture handler (a button click for
example).

JitsiTrackError.js

     = 'Firefox extension is not installed';
 TRACK_ERROR_TO_MESSAGE_MAP[JitsiTrackErrors.CHROME_EXTENSION_INSTALLATION_ERROR]
     = 'Failed to install Chrome extension';
+TRACK_ERROR_TO_MESSAGE_MAP[
+    JitsiTrackErrors.CHROME_EXTENSION_USER_GESTURE_REQUIRED]
+    = 'Failed to install Chrome extension - installations can only be initiated'
+        + ' by a user gesture.';
 TRACK_ERROR_TO_MESSAGE_MAP[JitsiTrackErrors.CHROME_EXTENSION_USER_CANCELED]
     = 'User canceled Chrome\'s screen sharing prompt';
 TRACK_ERROR_TO_MESSAGE_MAP[JitsiTrackErrors.CHROME_EXTENSION_GENERIC_ERROR]

JitsiTrackErrors.js

 export const CHROME_EXTENSION_INSTALLATION_ERROR
     = 'gum.chrome_extension_installation_error';
 
+/**
+ * This error indicates that the attempt to start screensharing was initiated by
+ * a script which did not originate in user gesture handler. It means that
+ * you should to trigger the action again in response to a button click for
+ * example.
+ * @type {string}
+ */
+export const CHROME_EXTENSION_USER_GESTURE_REQUIRED
+    = 'gum.chrome_extension_user_gesture_required';
+
 /**
  * An error which indicates that user canceled screen sharing window
  * selection dialog in jidesha extension for Chrome.

modules/RTC/ScreenObtainer.js

 const CHROME_NO_EXTENSION_ERROR_MSG // eslint-disable-line no-unused-vars
     = 'Could not establish connection. Receiving end does not exist.';
 
+/**
+ * The error message returned by chrome when the extension install action needs
+ * to be initiated by a user gesture.
+ * @type {string}
+ */
+const CHROME_USER_GESTURE_REQ_ERROR
+    = 'Chrome Web Store installations can only be initated by a user gesture.';
+
 /**
  * Handles obtaining a stream from a screen capture on different browsers.
  */
             = `Failed to install the extension from ${webStoreInstallUrl}`;
 
         logger.log(msg, e);
-        failCallback(new JitsiTrackError(
-            JitsiTrackErrors.CHROME_EXTENSION_INSTALLATION_ERROR,
-            msg));
+
+        const error
+            = e === CHROME_USER_GESTURE_REQ_ERROR
+                ? JitsiTrackErrors.CHROME_EXTENSION_USER_GESTURE_REQUIRED
+                : JitsiTrackErrors.CHROME_EXTENSION_INSTALLATION_ERROR;
+
+        failCallback(new JitsiTrackError(error, msg));
     },
 
     /* eslint-enable max-params */