Add UMA for header values in XHR's setRequestHeader() checked against RFC 7230

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

Index: third_party/WebKit/Source/core/frame/UseCounter.cpp
diff --git a/third_party/WebKit/Source/core/frame/UseCounter.cpp b/third_party/WebKit/Source/core/frame/UseCounter.cpp
index 9a33c917c57e17f1d88e7e259e0c3038cd87fed9..960641e2c86048c476fcf755581a040eee697a5e 100644
--- a/third_party/WebKit/Source/core/frame/UseCounter.cpp
+++ b/third_party/WebKit/Source/core/frame/UseCounter.cpp
@@ -930,7 +930,10 @@ String UseCounter::deprecationMessage(Feature feature)
         return "Request.context is deprecated and will be removed in M46 (see: https://www.chromestatus.com/feature/5534702526005248).";
 
     case HeaderValueNotMatchingRFC7230:
-        return "Header values not matching to RFC 7230, will be deprecated (see: https://www.chromestatus.com/feature/6457425448140800).";
+        return "Found a header value that doesn't match the field-content token production (containing invalid octets. see RFC 7230). We are experimenting whether we can throw for such header values (see: https://www.chromestatus.com/feature/6457425448140800).";
+
+    case XHRSetRequestHeaderAffectedByNormalization:
+        return "According to the Fetch Standard, setRequestHeader() must strip leading/trailing whitespaces, but it doesn't currently. We are experimenting whether we can make it conform to the spec (see: https://www.chromestatus.com/feature/6457425448140800). This script will be affected by the change.";

[kinuko, 2015/09/29 12:51:33]

"We are experimenting whether..." the intention of these sentences' are unclear,
do we expect developers to change the code so that it own't be affected by the
potential future change?  If so probably just writing it would be better.  
(E.g. "... doesn't currently.  The current behavior might be changed to conform
to the spec (see https://..../) and your code will be affected by the change.")

[hiroshige, 2015/09/29 13:21:38]

"Just experimenting" is somehow accurate: we want to collect stats, became not
sure whether we can actually do the change (due to expanding the scope to //net
as a result of discussions).

Should we turn this stats into UMAs (without warnings), if still exposing these
deprecation warnings to developers is confusing?

 
     // Features that aren't deprecated don't have a deprecation message.
     default: