Instrument window.pageXOffset and window.pageYOffset.

This is part of the identifiability study.

Bug: 973801
Change-Id: I5a7105c77ffbaa68d0f61384b9700ab1337332d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357612Reviewed-by: Nate Chapin <japhet@chromium.org>
Commit-Queue: Russ Hamilton <behamilton@google.com>
Cr-Commit-Position: refs/heads/master@{#799232}

third_party/blink/renderer/core/frame/window.idl

@@ -135,9 +135,9 @@
 
     // viewport scrolling
     [HighEntropy=Direct, MeasureAs=WindowScrollX, Replaceable] readonly attribute double scrollX;
-    [HighEntropy, MeasureAs=WindowPageXOffset, Replaceable] readonly attribute double pageXOffset;
+    [HighEntropy=Direct, MeasureAs=WindowPageXOffset, Replaceable] readonly attribute double pageXOffset;
     [HighEntropy=Direct, MeasureAs=WindowScrollY, Replaceable] readonly attribute double scrollY;
-    [HighEntropy, MeasureAs=WindowPageYOffset, Replaceable] readonly attribute double pageYOffset;
+    [HighEntropy=Direct, MeasureAs=WindowPageYOffset, Replaceable] readonly attribute double pageYOffset;
     void scroll(optional ScrollToOptions options = {});
     void scroll(unrestricted double x, unrestricted double y);
     void scrollTo(optional ScrollToOptions options = {});