Add a use counter and histogram to FileReaderSync.

third_party/WebKit/Source/core/frame/UseCounter.h

Index: third_party/WebKit/Source/core/frame/UseCounter.h
diff --git a/third_party/WebKit/Source/core/frame/UseCounter.h b/third_party/WebKit/Source/core/frame/UseCounter.h
index c5548f77b2f1c0a005483e313dc1bdcbd4a42700..a8fc6a8b24eabec4d64055fd14b545f1f8f8ad6c 100644
--- a/third_party/WebKit/Source/core/frame/UseCounter.h
+++ b/third_party/WebKit/Source/core/frame/UseCounter.h
@@ -1458,6 +1458,7 @@ class CORE_EXPORT UseCounter {
     HTMLIFrameElementAllowfullscreenAttributeSetAfterContentLoad = 1805,
     PointerEventSetCaptureOutsideDispatch = 1806,
     NotificationPermissionRequestedInsecureOrigin = 1807,
+    V8FileReaderSync_Constructor = 1808,

[kinuko, 2017/02/10 00:41:38]

nit: I think just using the class name as its prefix is more common
(V8FileReaderSync -> FileReaderSync)

[Marijn Kruisselbrink, 2017/02/10 17:42:53]

V8FileReaderSync is what the "Measure" idl extended attribute ends up using.
Anything else would require overriding it with MeasureAs

 
     // Add new features immediately above this line. Don't change assigned
     // numbers of any item, and don't reuse removed slots.