Fix crash when sending programmatic event to a Views-hosted browser window.

The |web_contents_| member was nullptr in CefBrowserPlatformDelegateNativeAura
when calling methods like SendKeyEvent from CefBrowserPlatformDelegateViews.

libcef/browser/native/browser_platform_delegate_native_aura.cc

@@ -225,6 +225,8 @@ int CefBrowserPlatformDelegateNativeAura::TranslateUiChangedButtonFlags(
 
 content::RenderWidgetHostViewAura*
 CefBrowserPlatformDelegateNativeAura::GetHostView() const {
+  if (!web_contents_)
+    return nullptr;
   return static_cast<content::RenderWidgetHostViewAura*>(
       web_contents_->GetRenderWidgetHostView());
 }