Standardize initStorageEvent in StorageEvent

initStorageEvent was standardized in
https://github.com/whatwg/html/pull/4063.

spec: https://html.spec.whatwg.org/multipage/webstorage.html#the-storageevent-interface

Bug: 697170
Change-Id: Ie08062686c82c22a068546e74ec89cbebf65afc8
Reviewed-on: https://chromium-review.googlesource.com/c/1411752
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Amos Lim <eui-sang.lim@samsung.com>
Cr-Commit-Position: refs/heads/master@{#623013}
4 files changed