[DO NOT LAND] Set @@toStringTag for DOM object prototypes

[DO NOT LAND] Set @@toStringTag for DOM object prototypes

[DO NOT LAND] Requires "--harmony-tostring" support enabled.

The behavior change is that ({}).toString.call() on a
prototype will return "[object HTMLElement]" rather than
"[object Object]". This also allows us to remove
calling SetClassName() on each instance, bringing the
DOM and ES6 closer together.

This is a willful violation of WebIDL, which explicitly
requires a @@toStringTag property on each *instance*
(e.g. "HTMLElement") and on each prototype
(e.g. "HTMLElementPrototype"). We do not believe
there is a web compat issue since Blink has never returned
a particular string, but we'll need to watch closely.

https://www.w3.org/Bugs/Public/show_bug.cgi?id=28244

BUG=239915

[jsbell, 2015-09-23 17:42:14]

migrated from https://codereview.chromium.org/1017733003/

[jsbell, 2016-01-07 17:49:31]

Description was changed from

==========
[DO NOT LAND] Set @@toStringTag for DOM object prototypes

[DO NOT LAND] Requires "--harmony-tostring" support enabled.

The behavior change is that ({}).toString.call() on a
prototype will return "[object HTMLElement]" rather than
"[object Object]". This also allows us to remove
calling SetClassName() on each instance, bringing the
DOM and ES6 closer together.

This is a willful violation of WebIDL, which explicitly
requires a @@toStringTag property on each *instance*
(e.g. "HTMLElement") and on each prototype
(e.g. "HTMLElementPrototype"). We do not believe
there is a web compat issue since Blink has never returned
a particular string, but we'll need to watch closely.

https://www.w3.org/Bugs/Public/show_bug.cgi?id=28244

BUG=239915
==========

to

==========
[DO NOT LAND] Set @@toStringTag for DOM object prototypes

[DO NOT LAND] Requires "--harmony-tostring" support enabled.

The behavior change is that ({}).toString.call() on a
prototype will return "[object HTMLElement]" rather than
"[object Object]". This also allows us to remove
calling SetClassName() on each instance, bringing the
DOM and ES6 closer together.

This is a willful violation of WebIDL, which explicitly
requires a @@toStringTag property on each *instance*
(e.g. "HTMLElement") and on each prototype
(e.g. "HTMLElementPrototype"). We do not believe
there is a web compat issue since Blink has never returned
a particular string, but we'll need to watch closely.

https://www.w3.org/Bugs/Public/show_bug.cgi?id=28244

BUG=239915
==========