This reverts commit 29c1eab9.

Reason for revert: Causes Blink test failures:
https://ci.chromium.org/p/v8/builders/ci/V8%20Blink%20Linux/4222

Original change's description:
> [builtins] Clean up the use of class_name / ES5 [[Class]]
> 
> Before ES2015, the ES spec had a [[Class]] internal slot for all
> objects, which Object.prototype.toString() would use to figure the
> returned string. Post-ES2015, the [[Class]] slot was removed in spec for
> all objects, with the @@toStringTag well-known symbol the proper way to
> change Object.prototype.toString() output.
> 
> At the time, spec-identical handling without the use of [[Class]] was
> implemented in V8 for all objects other than API objects, where issues
> with the Web IDL spec [1] prevented Blink, and hence V8, to totally
> migrate to @@toStringTag. However, since 2016 [2] Blink has been setting
> @@toStringTag on API class prototypes to manage the
> Object.prototype.toString() output, so the legacy [[Class]] handling in
> V8 has not been necessary for the past couple of years.
> 
> This CL removes the remaining legacy [[Class]] handling in
> Object.prototype.toString(), JSReceiver::class_name(), and
> GetConstructorName(). However, it does not remove the class_name field
> in FunctionTemplateInfo, as it is still used for the `name` property of
> created functions.
> 
> This CL also cleans up other places in the codebase that still reference
> [[Class]].
> 
> This change should have minimal impact on web-compatibility. For the
> change to be observable, a script must do one of the following:
> 
> 1. delete APIConstructor.prototype[Symbol.toStringTag];
> 2. Object.setPrototypeOf(apiObject, somethingElse);
> 
> Before this CL, these changes will not change the apiObject.toString()
> output. But after this CL, they will make apiObject.toString() show
> "[object Object]" (in the first case) or the @@toStringTag of the other
> prototype (in the latter case).
> 
> However, both are deemed unlikely. @@toStringTag is not well-known
> feature of JavaScript, nor does it get tampered much on API
> constructors. In the second case, setting the prototype of an API object
> would effectly render the object useless, as all its methods (including
> property getters/setters) would no longer be accessible.
> 
> Currently, @@toStringTag-based API object branding is not yet
> implemented by other browsers. This V8 bug in particular has been an
> impediment to standardizing toString behavior. Fixing this bug will
> unblock [3] and lead to a better Web IDL spec, and better toString()
> compatibility for all.
> 
> [1]: https://www.w3.org/Bugs/Public/show_bug.cgi?id=28244
> [2]: https://crrev.com/909c0d7d5a53c8526ded351683c65ea7d17531d4
> [3]: https://github.com/heycam/webidl/pull/357
> 
> Bug: chromium:793406
> Cq-Include-Trybots: luci.chromium.try:linux-rel
> Change-Id: Iceded24e37afa2646ec385d5018909f55b177f93
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2146996
> Commit-Queue: Timothy Gu <timothygu@chromium.org>
> Reviewed-by: Toon Verwaest <verwaest@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#67327}

TBR=verwaest@chromium.org,timothygu@chromium.org

Change-Id: I678d2ffc1064b1d1ddb62024cc23c6c41b216ef4
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:793406
Cq-Include-Trybots: luci.chromium.try:linux-rel
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2163956Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67349}