Instrument window.{scrollX,scrollY} attributes for identifiability

study.

These annotations will automatically generate code to measure how much
entropy these APIs leak about the users' cross-site identity.

Bug: 973801
Change-Id: Ie3f7bd8d9a77a2e6b75aa91e45ec4dd688bf6a67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357450
Auto-Submit: Chris Fredrickson <cfredric@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798409}

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

@@ -134,9 +134,9 @@
     [Affects=Nothing, HighEntropy=Direct, MeasureAs=WindowInnerHeight, Replaceable] readonly attribute long innerHeight;
 
     // viewport scrolling
-    [HighEntropy, MeasureAs=WindowScrollX, Replaceable] readonly attribute double scrollX;
+    [HighEntropy=Direct, MeasureAs=WindowScrollX, Replaceable] readonly attribute double scrollX;
     [HighEntropy, MeasureAs=WindowPageXOffset, Replaceable] readonly attribute double pageXOffset;
-    [HighEntropy, MeasureAs=WindowScrollY, Replaceable] readonly attribute double scrollY;
+    [HighEntropy=Direct, MeasureAs=WindowScrollY, Replaceable] readonly attribute double scrollY;
     [HighEntropy, MeasureAs=WindowPageYOffset, Replaceable] readonly attribute double pageYOffset;
     void scroll(optional ScrollToOptions options = {});
     void scroll(unrestricted double x, unrestricted double y);