OOP PDF: Fix PrintWebViewHelper test issues with OOP PDF enabled.

OOP PDF causes that the print-preview page to contain an iframe of the
PDF extension instead of directly including a plugin element. However,
the tests for PrintWebViewHelper use a fake print-preview page that
cannot contain an iframe of the extension because the necessary
infrastructure is not available in the test. This relaxes the
expectations so that in tests where the pdf element isn't an iframe,
that element is used instead.

BUG=303491

Review URL: https://codereview.chromium.org/800343004

Cr-Commit-Position: refs/heads/master@{#308450}

chrome/renderer/printing/print_web_view_helper.cc

@@ -901,14 +901,13 @@ void PrintWebViewHelper::OnPrintForPrintPreview(
     return;
   }
 
-  // The out-of-process plugin element is nested within a frame.
+  // The out-of-process plugin element is nested within a frame. In tests, there
+  // may not be an iframe containing the out-of-process plugin, so continue with
+  // the element with ID "pdf-viewer" if it isn't an iframe.
   blink::WebLocalFrame* plugin_frame = pdf_element.document().frame();
   blink::WebElement plugin_element = pdf_element;
-  if (switches::OutOfProcessPdfEnabled()) {
-    if (!pdf_element.hasHTMLTagName("iframe")) {
-      NOTREACHED();
-      return;
-    }
+  if (switches::OutOfProcessPdfEnabled() &&
+      pdf_element.hasHTMLTagName("iframe")) {
     plugin_frame = blink::WebLocalFrame::fromFrameOwnerElement(pdf_element);
     plugin_element = GetPdfElement(plugin_frame);
     if (plugin_element.isNull()) {