Avoid crash if the webcontents is opened in views::WebView

chrome::FindBrowserWithWebContents returns nullptr when the webcontents
opened in a views::WebView

Change-Id: I6465bbd83efe7cef23568ab8026c5eb96990e769
Reviewed-on: https://chromium-review.googlesource.com/1056547Reviewed-by: Jun Cai <juncai@chromium.org>
Commit-Queue: Ganggui Tang <gogerald@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558406}

chrome/browser/usb/web_usb_chooser_service_desktop.cc

@@ -33,5 +33,6 @@ void WebUsbChooserServiceDesktop::ShowChooser(
   auto* web_contents =
       content::WebContents::FromRenderFrameHost(render_frame_host());
   Browser* browser = chrome::FindBrowserWithWebContents(web_contents);
+  if (browser)
     bubble_ = browser->GetBubbleManager()->ShowBubble(std::move(delegate));
 }