Delete PrintingUIWebContentsObserver on UI thread.

chrome/browser/printing/printing_ui_web_contents_observer.cc

Index: chrome/browser/printing/printing_ui_web_contents_observer.cc
diff --git a/chrome/browser/printing/printing_ui_web_contents_observer.cc b/chrome/browser/printing/printing_ui_web_contents_observer.cc
index 311d62192f9849d2cea8b21baa2f6c534213be74..541e93392747573e7d503da088789babc2554765 100644
--- a/chrome/browser/printing/printing_ui_web_contents_observer.cc
+++ b/chrome/browser/printing/printing_ui_web_contents_observer.cc
@@ -13,6 +13,10 @@ PrintingUIWebContentsObserver::PrintingUIWebContentsObserver(
   DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
 }
 
+PrintingUIWebContentsObserver::~PrintingUIWebContentsObserver() {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
+}
+
 gfx::NativeView PrintingUIWebContentsObserver::GetParentView() {
   DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
   return web_contents() ? web_contents()->GetNativeView() : NULL;