In ES2015, Date.prototype.toGMTString is simply an alias of
Date.prototype.toUTCString, so it has the same identity as a function and
doesn't have its own name. Firefox has already shipped this behavior.
Previously, we copied JSC behavior by making it a separate function.
This change makes an addition test262 test pass.
BUG=v8:4708
LOG=Y
R=adamk
Review URL: https://codereview.chromium.org/1709373002
Cr-Commit-Position: refs/heads/master@{#34131}
