Add use counters for lots of DOM APIs.

third_party/WebKit/Source/core/events/MouseEvent.idl

Index: third_party/WebKit/Source/core/events/MouseEvent.idl
diff --git a/third_party/WebKit/Source/core/events/MouseEvent.idl b/third_party/WebKit/Source/core/events/MouseEvent.idl
index 8cbea08d0d22503267a1d651821d227b829d27d2..0d7e8f7fe44fcd0fab845512f8be6cd2b633b37a 100644
--- a/third_party/WebKit/Source/core/events/MouseEvent.idl
+++ b/third_party/WebKit/Source/core/events/MouseEvent.idl
@@ -32,9 +32,9 @@
     readonly attribute boolean          altKey;
     readonly attribute boolean          metaKey;
     readonly attribute short            button;
-    readonly attribute unsigned short   buttons;
+    [Measure] readonly attribute unsigned short   buttons;
     readonly attribute EventTarget?     relatedTarget;
-    boolean getModifierState(DOMString keyArg);
+    [Measure] boolean getModifierState(DOMString keyArg);
 
     // https://w3c.github.io/uievents/#idl-interface-MouseEvent-initializers
     // TODO(foolip): None of the initMouseEvent() arguments should be optional.
@@ -54,8 +54,8 @@
                                                         [Default=Undefined] optional unsigned short button,
                                                         [Default=Undefined] optional EventTarget? relatedTarget);
 
-    readonly attribute double pageX;
-    readonly attribute double pageY;
+    [Measure] readonly attribute double pageX;
+    [Measure] readonly attribute double pageY;
     [MeasureAs=MouseEventX] readonly attribute double x;
     [MeasureAs=MouseEventY] readonly attribute double y;
     [MeasureAs=MouseEventOffsetX] readonly attribute double offsetX;
@@ -68,7 +68,7 @@
 
     // Canvas Hit Regions
     // https://html.spec.whatwg.org/multipage/scripting.html#MouseEvent-partial
-    [RuntimeEnabled=ExperimentalCanvasFeatures] readonly attribute DOMString? region;
+    [RuntimeEnabled=ExperimentalCanvasFeatures, Measure] readonly attribute DOMString? region;
 
     // Non-standard
     [MeasureAs=MouseEventFromElement] readonly attribute Node fromElement;