Remove the private property "ErrorEventError"

This CL removes a private property.
We do not have to have both of the private property of
"ErrorEventError" and |error_|, the
TraceWrapperV8Reference<v8::Value> in blink::ErrorEvent.

Also, the error attribute passed to listener should be initialized to
null for dedicated workers.
https://html.spec.whatwg.org/multipage/workers.html#runtime-script-errors-2
This CL modifies some tests regarding this too.

Bug: 872138
Change-Id: I497434d04c30f222ea8847239edf4e9a0ee97b19
Reviewed-on: https://chromium-review.googlesource.com/1233080
Commit-Queue: Yuki Yamada <yukiy@google.com>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593824}
14 files changed