chrome: Support unload handlers with TryCloseBrowser

TryCloseBrowser should potentially trigger JavaScript unload handlers
and return false. Add CefBrowserHost::IsReadyToBeClosed for detecting
mandatory close state. Enable, for Chrome style browsers, LifeSpanTest
that don't require DoClose(). Update related documentation.

libcef/browser/browser_host_base.h

@@ -218,6 +218,7 @@ class CefBrowserHostBase : public CefBrowserHost,
   double GetZoomLevel() override;
   void SetZoomLevel(double zoomLevel) override;
   bool HasView() override;
+  bool IsReadyToBeClosed() override;
   void SetFocus(bool focus) override;
   void RunFileDialog(FileDialogMode mode,
                      const CefString& title,