GitLab

- Adds Widget::SetFullscreenAuxiliary(), which sets
  NSWindowCollectionBehaviorFullScreenAuxiliary and lets the dragging
  window exist in another window's fullscreen space.

- Sets that flag when tab dragging starts, clears it when dragging ends, and
  makes the new window fullscreen if the source was fullscreen.

Bug: 883910
Change-Id: I8889135320a0fb26abff9b546c5f5444ba6f4486
Reviewed-on: https://chromium-review.googlesource.com/c/1484491
Commit-Queue: Sidney San Martín <sdy@chromium.org>
Auto-Submit: Sidney San Martín <sdy@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636278}