Move Get/SetScrollOffset methods from WebFrame to WebLocalFrame.

components/printing/renderer/print_web_view_helper.cc

Index: components/printing/renderer/print_web_view_helper.cc
diff --git a/components/printing/renderer/print_web_view_helper.cc b/components/printing/renderer/print_web_view_helper.cc
index fba82e03c92b2852485624fed2758c51c93de2d0..1e022dac8a952f564bbf891c1930346cd994270b 100644
--- a/components/printing/renderer/print_web_view_helper.cc
+++ b/components/printing/renderer/print_web_view_helper.cc
@@ -760,7 +760,7 @@ void PrepareFrameAndViewForPrint::ResizeForPrinting() {
   blink::WebView* web_view = frame_.view();
   if (blink::WebFrame* web_frame = web_view->MainFrame()) {
     if (web_frame->IsWebLocalFrame())

[dcheng, 2017/06/16 23:52:20]

I wonder about this code. Isn't it always the case that it's a main frame? See
https://cs.chromium.org/chromium/src/components/printing/renderer/print_web_v...

(I'm not really sure though, as I don't really understand what's going on here)

[Łukasz Anforowicz, 2017/06/19 23:58:08]

Agreed that |frame_| is always a local frame.  I think the question is: Is
|frame_->view()->MainFrame()| always a local frame.

I looked for code paths that can set the value of the
PrepareFrameAndViewForPrint::frame_ field.  This can be set via:

1. PrepareFrameAndViewForPrint::CopySelection (in this case |frame_| is indeed
the main frame)

2. PrepareFrameAndViewForPrint's constructor (in this case |frame_| is not
necessarily the main frame, and therefore the main frame can be remote).


I've confirmed in a manual test that CHECK(web_frame->IsWebLocalFrame()) would
fail here.  Repro steps:
1. Add CHECK
2. Launch chrome with --site-per-process
3. Navigate to https://csreis.github.io/tests/cross-site-iframe.html
4. Click "Go cross-site (complex page)" button
5. Inspect the subframe
6. Run window.print() in the subframe

-      prev_scroll_offset_ = web_frame->GetScrollOffset();
+      prev_scroll_offset_ = web_frame->ToWebLocalFrame()->GetScrollOffset();
   }
   prev_view_size_ = web_view->Size();
 
@@ -862,7 +862,7 @@ void PrepareFrameAndViewForPrint::RestoreSize() {
   web_view->Resize(prev_view_size_);
   if (blink::WebFrame* web_frame = web_view->MainFrame()) {
     if (web_frame->IsWebLocalFrame())
-      web_frame->SetScrollOffset(prev_scroll_offset_);
+      web_frame->ToWebLocalFrame()->SetScrollOffset(prev_scroll_offset_);

[dcheng, 2017/06/16 23:52:20]

Ditto: maybe I am misunderstanding this code?

   }
 }