This used to exist in the spec but it was refactored into WindowOrWorkerGlobalScope. There should be no observable behavior change in moving things there.

This issue has been Available for over a year. If it's no longer important or seems unlikely to be fixed, please consider closing it out. If it is important, please re-triage the issue.

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The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/93b8d80ed987603ed0d5851400f6d30d4d9806da

commit 93b8d80ed987603ed0d5851400f6d30d4d9806da
Author: Andrew Comminos <acomminos@fb.com>
Date: Thu Nov 29 03:43:11 2018

Unify WindowBase64 into WindowOrWorkerGlobalScope.

The HTML5 spec recently refactored WindowBase64 into
WindowOrWorkerGlobalScope, removing the need to reflect this in our IDL.
{btoa,atob} implementations have been merged into
window_or_worker_global_scope.{cc,h}.

This CL has no behavior changes.

Bug:  701498 
Change-Id: Ib40b30e166ef3e4c0c92791f3ffecbb7bd1bed0b
Reviewed-on: https://chromium-review.googlesource.com/c/1351141
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Andrew Comminos <acomminos@fb.com>
Cr-Commit-Position: refs/heads/master@{#612020}
[modify] https://crrev.com/93b8d80ed987603ed0d5851400f6d30d4d9806da/third_party/blink/renderer/core/core_idl_files.gni
[modify] https://crrev.com/93b8d80ed987603ed0d5851400f6d30d4d9806da/third_party/blink/renderer/core/frame/BUILD.gn
[modify] https://crrev.com/93b8d80ed987603ed0d5851400f6d30d4d9806da/third_party/blink/renderer/core/frame/dom_window.h
[delete] https://crrev.com/1b39ce73c483efc6792ed5b2dbfcf3fc6649be36/third_party/blink/renderer/core/frame/dom_window_base64.cc
[delete] https://crrev.com/1b39ce73c483efc6792ed5b2dbfcf3fc6649be36/third_party/blink/renderer/core/frame/dom_window_base64.h
[modify] https://crrev.com/93b8d80ed987603ed0d5851400f6d30d4d9806da/third_party/blink/renderer/core/frame/window.idl
[delete] https://crrev.com/1b39ce73c483efc6792ed5b2dbfcf3fc6649be36/third_party/blink/renderer/core/frame/window_base64.idl
[modify] https://crrev.com/93b8d80ed987603ed0d5851400f6d30d4d9806da/third_party/blink/renderer/core/frame/window_or_worker_global_scope.cc
[modify] https://crrev.com/93b8d80ed987603ed0d5851400f6d30d4d9806da/third_party/blink/renderer/core/frame/window_or_worker_global_scope.h
[modify] https://crrev.com/93b8d80ed987603ed0d5851400f6d30d4d9806da/third_party/blink/renderer/core/frame/window_or_worker_global_scope.idl
[modify] https://crrev.com/93b8d80ed987603ed0d5851400f6d30d4d9806da/third_party/blink/renderer/core/workers/worker_global_scope.h
[modify] https://crrev.com/93b8d80ed987603ed0d5851400f6d30d4d9806da/third_party/blink/renderer/core/workers/worker_global_scope.idl