Bug 209119 Need WebViewer provide cancel function
Need WebViewer provide a cancel function, this will be used to cancel current report execution during designer preview.
viewer/org.eclipse.birt.report.viewer/src/org/eclipse/birt/report/viewer/utilities/WebViewer.java
{
	/**
	 * HTML format name
	 */
	public final static String HTML = "html"; 
	public final static String HTM = "htm"; 
	/**
	 * PDF format name
	 */
	public final static String PDF = "pdf"; 
	/**
	 * DOC format name
	 */
	public final static String DOC = "doc"; 
	/**
	 * POSTSCRIPT format name
	 */
	public final static String POSTSCRIPT = "postscript"; 
	/**
	 * Report extension
	 */
	public static final String REPORT_DOCUMENT_FILE = ".rptdocument"; 
	/**
	 * Birt web viewer plugin id
	 */