Revert of Remove RegExp.prototype.source getter compat workaround (patchset #2 id:20001 of https://codereview.chromium.org/1837843002/ )

Reason for revert:
TC39 decided that this compatibility fix should be standardized.

Original issue's description:
> Remove RegExp.prototype.source getter compat workaround
>
> The getter RegExp.prototype.source is specified in ES2015 to throw when
> called on a non-RegExp instance, such as RegExp.prototype. We had previously
> put in a compatibility workaround for all RegExp getters to make them
> throw on access specifically with RegExp.prototype as the receiver; however,
> we only have evidence that this is needed for properties other than source.
> This patch removes the compatibility workaround for get RegExp.prototype.source
> and gives it semantics precisely as per the ES2015 specification.
>
> R=adamk
> BUG=chromium:581577,v8:4827
> LOG=Y
>
> Committed: https://crrev.com/80803aa89e31839b8f73959776fa7e1923c6b461
> Cr-Commit-Position: refs/heads/master@{#35086}

R=adamk@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=chromium:581577,v8:4827
LOG=Y

Review URL: https://codereview.chromium.org/1847783003

Cr-Commit-Position: refs/heads/master@{#35180}