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Adapt to https://github.com/heycam/webidl/pull/378 ("Re-align DOMException
objects with what is implemented").

We were reimplementing toString() in DOMException because of WebKit
r29058 ("Acid3 expects ExeceptionCode constants to be defined on
DOMException objects") from almost 10 years ago. A lot has happened since,
and we can (and should) just use the toString() implementation from
ECMAScript's %ErrorProtoype% (which is explicitly mandated to be in
DOMException's inheritance chain by the WebIDL spec).

Contrary to what's originally described in bug 556950, we do throw an
exception when DOMException.prototype.toString() is called directly: the
WebIDL spec now expects it to, and
https://github.com/w3c/web-platform-tests/pull/6361 tests this behavior.

Additionally, we've changed the way DOMException inherits from
%ErrorPrototype%: instead of doing it in V8PerContextData, we now do it in
V8DOMException::installV8DOMExceptionTemplate(), as the former had problems
with (i)frames detached from the DOM and toString() would just call
Object.prototype.toString() instead.

The only user-visible part of the change is that "toString" is no longer
part of DOMException.prototype's own properties; the error message format is
exactly the same in most cases (the exact steps are described in
https://tc39.github.io/ecma262/#sec-error.prototype.tostring).

Finally, part of http/tests/plugins/cross-frame-object-access.html's
output will change from:
    "Error: Uncaught [object DOMException]"
to
    "Error: Uncaught"
This is tricky because it involves PPAPI and its separate process, but
basically the plugin in an iframe is trying to access top.location.href,
Blink is throwing a SecurityError, but the error message is sent truncated
to PPAPI. The message is truncated because V8 is calling its
NoSideEffectsErrorToString() when creating the message, and this one does
not use the message/name accessors we install, leading to an empty message
(it looked slightly better before because we the presence of our own
toString() caused Object::NoSideEffectsToString() to choose a different
albeit still wrong code path). Blink's handling of this is fine, as the
code in V8Initializer takes care of extracting the name and error message
from the DOMException V8 object that threw the exception.

Bug: 556950, 737497
Change-Id: I9d81edca1de903364bb1aca5950c313885c5964a
Reviewed-on: https://chromium-review.googlesource.com/558904
Commit-Queue: Raphael Kubo da Costa (rakuco) <raphael.kubo.da.costa@intel.com>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485960}