Remove last use of WebFrame::isLoading()

All other uses are from a WebLocalFrame, and the concept is local-specific.
This will enable us to move it to WebLocalFrame in the blink repo.

BUG=

Review URL: https://codereview.chromium.org/557483002

Cr-Commit-Position: refs/heads/master@{#293869}

chrome/renderer/printing/print_web_view_helper.cc

@@ -753,7 +753,7 @@ void PrepareFrameAndViewForPrint::RestoreSize() {
 }
 
 void PrepareFrameAndViewForPrint::FinishPrinting() {
-  blink::WebFrame* frame = frame_.GetFrame();
+  blink::WebLocalFrame* frame = frame_.GetFrame();
   if (frame) {
     blink::WebView* web_view = frame->view();
     if (is_printing_started_) {