Instrument PaintWorkletGlobalScope.devicePixelRatio for identifiability

study.

This IDL annotation will automatically generate code to measure the
entropy that can be used to establish users' cross-site identity using
this API.

Bug: 973801
Change-Id: Ib93b5421d0f63ff0158d5cf9b46d22dacda3c2a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2356909
Auto-Submit: Chris Fredrickson <cfredric@google.com>
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Chris Fredrickson <cfredric@google.com>
Cr-Commit-Position: refs/heads/master@{#799234}

third_party/blink/renderer/modules/csspaint/paint_worklet_global_scope.idl

@@ -8,7 +8,7 @@
     Exposed=PaintWorklet,
     Global=(Worklet,PaintWorklet)
 ] interface PaintWorkletGlobalScope : WorkletGlobalScope {
-    [Measure] readonly attribute unrestricted double devicePixelRatio;
+    [HighEntropy=Direct, Measure] readonly attribute unrestricted double devicePixelRatio;
     [Measure, RaisesException, CallWith=ScriptState] void registerPaint(DOMString name, NoArgumentConstructor paintCtor);
 };