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

Review URL: https://codereview.chromium.org/1837843002

Cr-Commit-Position: refs/heads/master@{#35086}