Add comments about counter availability in M46 and M47 branches.

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 b29ccf9ba757eb7aa79dd3d27830666e4b0c55c5..ccaf40425f9836d962293db62aee3c7a39079458 100644
--- a/third_party/WebKit/Source/core/frame/UseCounter.h
+++ b/third_party/WebKit/Source/core/frame/UseCounter.h
@@ -813,6 +813,8 @@ public:
         CSSAnimationsStackedNeutralKeyframe = 937,
         ReadingCheckedInClickHandler = 938,
         FlexboxIntrinsicSizeAlgorithmIsDifferent = 939,
+        // The above items are available in M46 branch.
+
         HTMLImportsHasStyleSheets = 940,
         WebkitTextInClipProperty = 941,
         WebkitTextInColorProperty = 942,
@@ -838,6 +840,8 @@ public:
         CredentialManagerGetWithoutUI = 962,
         CredentialManagerStore = 963,
         CredentialManagerRequireUserMediation = 964,
+        // The above items are available in M47 branch.
+
         RequestAutocomplete = 965,
 
         // Add new features immediately above this line. Don't change assigned