Use WebLocalFrame::isLoading(), not WebFrame::isLoading(), in print preview.

This was changed in the normal chromium print_preview_web_helper.cc in
02cfd5cc, but I missed the need to change it
in android's print_preview_web_helper.cc. This will allow isLoading() to live
solely on WebLocalFrame, as it doesn't make sense in a remote frame.

BUG=

Review URL: https://codereview.chromium.org/556923002

Cr-Commit-Position: refs/heads/master@{#294022}

android_webview/renderer/print_web_view_helper.cc

@@ -724,7 +724,7 @@ 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_) {