Adds an error for user canceling the window choosing window/desktop when sharing desktop.

JitsiTrackErrors.js

     FIREFOX_EXTENSION_NEEDED: "gum.firefox_extension_needed",
     CHROME_EXTENSION_INSTALLATION_ERROR:
         "gum.chrome_extension_installation_error",
+    CHROME_EXTENSION_USER_CANCELED:
+        "gum.chrome_extension_user_canceled",
     GENERAL: "gum.general"
 };

modules/RTC/ScreenObtainer.js

                     failCallback,
                     {desktopStream: response.streamId});
             } else {
+                // As noted in Chrome Desktop Capture API:
+                // If user didn't select any source (i.e. canceled the prompt)
+                // then the callback is called with an empty streamId.
+                if(response.streamId === "")
+                {
+                    failCallback({
+                        type: "jitsiError",
+                        errorObject:
+                            JitsiTrackErrors.CHROME_EXTENSION_USER_CANCELED
+                    });
+                    return;
+                }
+
                 failCallback("Extension failed to get the stream");
             }
         }