Disable 'Save as HTML-only' on supervised accounts

When trying to access a blocked webpage, the child user can press
Ctrl+S and save the error page (the one with the "Ask your Parent"
message).

If the child chooses to save the page as HTML-only (.html), the
downloaded source code is the one from the blocked page. The child
can then open the page locally and see the blocked content.

As a workaround, we are temporarily disabling the HTML-only download
of the blocked page for supervised users. They can still save the
page as MHTML, which works as expected.

AFTER THIS CL
Normal user (unaffected):
    https://screenshot.googleplex.com/kipKMUOKA6r
Supervised user (normal page):
    https://screenshot.googleplex.com/r3hQ6E1edx3
Supervised user (blocked page):
    https://screenshot.googleplex.com/Jqba7rhiOf2

Bug: 928323
Change-Id: I7a82cfb9194a14070910716848228ddb1faeedff
Reviewed-on: https://chromium-review.googlesource.com/c/1460876
Commit-Queue: Felipe Cerqueira <[email protected]>
Reviewed-by: Carlos IL <[email protected]>
Reviewed-by: Michael Giuffrida <[email protected]>
Reviewed-by: Min Qin <[email protected]>
Cr-Commit-Position: refs/heads/master@{#633691}
diff --git a/chrome/browser/download/save_package_file_picker.h b/chrome/browser/download/save_package_file_picker.h
index 628089e..90e898c7 100644
--- a/chrome/browser/download/save_package_file_picker.h
+++ b/chrome/browser/download/save_package_file_picker.h
@@ -38,6 +38,7 @@
                     void* unused_params) override;
   void FileSelectionCanceled(void* unused_params) override;
 
+  bool ShouldSaveAsOnlyHTML(content::WebContents* web_contents) const;
   bool ShouldSaveAsMHTML() const;
 
   // Used to look up the renderer process for this request to get the context.