Delete PrintingUIWebContentsObserver on UI thread.

BUG=374321
TBR=noamsml

Review URL: https://codereview.chromium.org/490763003

Cr-Commit-Position: refs/heads/master@{#290785}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290785 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/printing/print_job_worker.cc

@@ -105,6 +105,8 @@ void PrintJobWorker::GetSettings(
                               document_page_count,
                               has_selection)));
   } else {
+    BrowserThread::DeleteSoon(
+        BrowserThread::UI, FROM_HERE, web_contents_observer.release());
     BrowserThread::PostTask(
         BrowserThread::UI, FROM_HERE,
         base::Bind(&HoldRefCallback, make_scoped_refptr(owner_),

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;

chrome/browser/printing/printing_ui_web_contents_observer.h

@@ -14,6 +14,7 @@
 class PrintingUIWebContentsObserver : public content::WebContentsObserver {
  public:
   explicit PrintingUIWebContentsObserver(content::WebContents* web_contents);
+  virtual ~PrintingUIWebContentsObserver();
 
   // Return the parent NativeView of the observed WebContents.
   gfx::NativeView GetParentView();