Avoid potential crash on shutdown under Mac

We can't wait for OnBeforeClose() to be called in the dtor there, or,
IOW, we can't count on it being called before the object is destroyed
and hence we can't update the flag in it neither.

Don't do it and just hope that the ClientHandler object always gets
destroyed soon after wxWebViewChromium itself anyhow because it's only
referenced by an autorelease pool.

src/common/webview_chromium.cpp

@@ -534,6 +534,12 @@ wxWebViewChromium::~wxWebViewChromium()
             }
         }
         //else: we're going to crash on shutdown, but what else can we do?
+#elif defined(__WXOSX__)
+        // There doesn't seem to be any way to force OnBeforeClose() to be
+        // called from here under Mac as it's referenced by an autorelease pool
+        // in the outer frame, so just return and count on that pool dtor
+        // really destroying the object before CefShutdown() is called.
+        wxUnusedVar(handle);
 #endif
     }
 
@@ -604,6 +610,8 @@ void wxWebViewChromium::OnSize(wxSizeEvent& event)
 {
     event.Skip();
 
+    // Under Mac we don't have to do anything to resize the browser window.
+#ifndef __WXOSX__
     const auto handle = m_clientHandler ? m_clientHandler->GetWindowHandle() : 0;
     if ( !handle )
         return;
@@ -618,6 +626,7 @@ void wxWebViewChromium::OnSize(wxSizeEvent& event)
 
     ::XResizeWindow(wxGetX11Display(), handle, size.x, size.y);
 #endif
+#endif // !__WXOSX__
 }
 
 void wxWebViewChromium::SetPageSource(const wxString& pageSource)
@@ -1015,7 +1024,11 @@ void ClientHandler::OnBeforeClose(CefRefPtr<CefBrowser> browser)
 {
     TRACE_CEF_FUNCTION();
 
+    // Under Mac the web view and its data might be already destroyed, so don't
+    // touch them there, see Mac-specific comment in wxWebViewChromium dtor.
+#ifndef __WXOSX__
     m_webview.m_implData->m_calledOnBeforeClose = true;
+#endif
 
     if ( browser->GetIdentifier() == m_browserId )
     {