Bug 299438 [CommonNavigator] CNF viewer state non properly reset when NCEs are activated or deactivated
The map of the objects to the NCEs should be reset whenever the NCEs associated with the viewer to change to avoid having stale label information. This has shown up when looking at the CNF tests that change the viewer.
bundles/org.eclipse.ui.navigator/src/org/eclipse/ui/internal/navigator/NavigatorContentService.java
*/
	public final String getViewerId() {
		return viewerDescriptor.getViewerId();
	}
	/**
	 * 
	 * @param aDescriptorKey
	 *            A descriptor
	 * @return The cached NavigatorContentExtension from the descriptor
	 */
	public final NavigatorContentExtension getExtension(
			INavigatorContentDescriptor aDescriptorKey) {
		return getExtension(aDescriptorKey, true);
	}
	/**
	 * 
	 * @param aDescriptorKey
	 * @param toLoadIfNecessary
	 *            True if the extension should be loaded if it is not already.
	 * @return The instance of the extension for the given descriptor key.
	 */
	public final NavigatorContentExtension getExtension(
			INavigatorContentDescriptor aDescriptorKey,
			boolean toLoadIfNecessary) {
		
		NavigatorContentExtension extension = (NavigatorContentExtension) contentExtensions
				.get(aDescriptorKey);
		if (extension != null || !toLoadIfNecessary) {
			return extension;
		}