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 b78eab5f47bd63b7510a6bc5bd40ef2df1c29e66..9b7fb0765edf4365798fd972e7b4e128bb7a25d2 100644
--- a/third_party/WebKit/Source/core/frame/UseCounter.h
+++ b/third_party/WebKit/Source/core/frame/UseCounter.h
@@ -1456,6 +1456,7 @@ class CORE_EXPORT UseCounter {
     HTMLIFrameElementAllowfullscreenAttributeSetAfterContentLoad = 1805,
     PointerEventSetCaptureOutsideDispatch = 1806,
     NotificationPermissionRequestedInsecureOrigin = 1807,
+    V8FileReaderSync_Constructor = 1808,

[Mark P, 2017/02/10 22:56:42]

Please resync and rebase.  And also don't forget to run
update_use_counter_feature_enum.py

[Marijn Kruisselbrink, 2017/02/10 23:17:28]

Yes? Of course I'll make sure the patch still applies before it lands?

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