Ship RegExp subclassing

This patch ships the first part of RegExp subclassing--defining
Symbol.{match,replace,search,split}, but keeping their original
definitions which are restricted to a RegExp receiver and do not
call out to the core 'exec' method. This is being shipped separately
because the two sets of extension points are separate features with
separate functionality. The amount of behavior which is held behind
the flag is very small, just exposing the symbols as properties of
Symbol--the behavior that the String methods call out to these Symbol
properties has already been shipping unflagged.

R=yangguo@chromium.org
BUG=v8:4305,v8:4343,v8:4344,v8:4345
LOG=Y
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_rel_ng;tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1652793002

Cr-Commit-Position: refs/heads/master@{#33629}

src/flag-definitions.h

@@ -217,8 +217,7 @@ DEFINE_IMPLICATION(es_staging, move_object_start)
 // Features that are complete (but still behind --harmony/es-staging flag).
 #define HARMONY_STAGED(V)                                   \
   V(harmony_regexp_lookbehind, "harmony regexp lookbehind") \
-  V(harmony_unicode_regexps, "harmony unicode regexps")     \
-  V(harmony_regexp_subclass, "harmony regexp subclassing")
+  V(harmony_unicode_regexps, "harmony unicode regexps")
 
 // Features that are shipping (turned on by default, but internal flag remains).
 #define HARMONY_SHIPPING(V)                                               \
@@ -235,8 +234,8 @@ DEFINE_IMPLICATION(es_staging, move_object_start)
   V(harmony_sloppy_let, "harmony let in sloppy mode")                     \
   V(harmony_sloppy_function, "harmony sloppy function block scoping")     \
   V(harmony_proxies, "harmony proxies")                                   \
-  V(harmony_reflect, "harmony Reflect API")
-
+  V(harmony_reflect, "harmony Reflect API")                               \
+  V(harmony_regexp_subclass, "harmony regexp subclassing")
 
 // Once a shipping feature has proved stable in the wild, it will be dropped
 // from HARMONY_SHIPPING, all occurrences of the FLAG_ variable are removed,