This patch stages 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 staged 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

Review URL: https://codereview.chromium.org/1637703003

Cr-Commit-Position: refs/heads/master@{#33534}