It turns out that some old polyfill library uses
RegExp.prototype.flags as a way of feature testing. It's not clear
how widespread this is. For now, as a minimal workaround, we can
return undefined from getters like RegExp.prototype.global when
the receiver is RegExp.prototype. This patch implements that strategy
but omits a UseCounter to make backports easier.

R=adamk
CC=yangguo@chromium.org
BUG=chromium:581577
LOG=Y
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_rel_ng;tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1640803003

Cr-Commit-Position: refs/heads/master@{#34201}