Reland of Add web compat workarounds for ES2015 RegExp semantics (patchset #3 id:40001 of https://codereview.chromium.org/1543723002/ )

Unexpectedly, websites depend on doing feature testing with
RegExp.prototype.sticky and browser testing with RegExp.prototype.toString().
ES2015 newly throws exceptions for both of these. In order to enable shipping
new ES2015 semantics, this patch puts in narrow workarounds for those two
cases, keeping their old behavior. UseCounters are added for how often
those particular cases come up, so we can see if it can be deprecated.

This reland replaces problematic legacy const usage with var, to
avoid issues with nosnap builds.

R=yangguo
CC=bmeurer
BUG=v8:4637,v8:4617
LOG=Y
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_rel_ng;tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1545633002

Cr-Commit-Position: refs/heads/master@{#33002}