Reland Stage --harmony-function-tostring"

Relanding now that v8:6190 has been fixed

BUG=v8:4958
R=adamk@chromium.org, littledan@chromium.org, jwolfe@igalia.com

Change-Id: I2732dbf96c5f9f899cee826dd2fdc621098a87e5
Reviewed-on: https://chromium-review.googlesource.com/466226Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Commit-Queue: Daniel Ehrenberg <littledan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44351}

src/flag-definitions.h

@@ -201,7 +201,6 @@ DEFINE_IMPLICATION(es_staging, harmony)
   V(harmony_do_expressions, "harmony do-expressions")                   \
   V(harmony_regexp_dotall, "harmony regexp dotall flag")                \
   V(harmony_regexp_property, "harmony unicode regexp property classes") \
-  V(harmony_function_tostring, "harmony Function.prototype.toString")   \
   V(harmony_class_fields, "harmony public fields in class literals")    \
   V(harmony_async_iteration, "harmony async iteration")                 \
   V(harmony_dynamic_import, "harmony dynamic import")                   \
@@ -209,6 +208,7 @@ DEFINE_IMPLICATION(es_staging, harmony)
 
 // Features that are complete (but still behind --harmony/es-staging flag).
 #define HARMONY_STAGED(V)                                                \
+  V(harmony_function_tostring, "harmony Function.prototype.toString")    \
   V(harmony_regexp_lookbehind, "harmony regexp lookbehind")              \
   V(harmony_regexp_named_captures, "harmony regexp named captures")      \
   V(harmony_restrictive_generators,                                      \

test/mjsunit/messages.js

@@ -371,7 +371,7 @@ test(function() {
 // kParenthesisInArgString
 test(function() {
   new Function(")", "");
-}, "Function arg string contains parenthesis", SyntaxError);
+}, "Arg string terminates parameters early", SyntaxError);
 
 // === ReferenceError ===