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This reverts commit 002dfaef.

Reason for revert: https://bugs.chromium.org/p/chromium/issues/detail?id=903706#c1

Original change's description:
> Mac: Disallow dragging tab into blocked tabstrip
> 
> This is a really longwinded way for me to fix some NOTIMPLEMENTED spam,
> though I'm pretty sure the thing that happens right now when you open
> a certificate prompt and drag a tab into the parent window is bad.
> 
> This introduces a new method because we're not sure if it matches the
> Aura semantics, and this is better than the nothing we were doing previously.
> 
> Bug: 825834
> Change-Id: I316ca4fbaa7a27dacb09bce0866203b79567016d
> Reviewed-on: https://chromium-review.googlesource.com/c/1315956
> Commit-Queue: Leonard Grey <lgrey@chromium.org>
> Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#606619}

TBR=ellyjones@chromium.org,lgrey@chromium.org

Change-Id: If2a6470657f160a0e7fa2fd9978078ab46ae048a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 825834
Reviewed-on: https://chromium-review.googlesource.com/c/1329341Reviewed-by: Leonard Grey <lgrey@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606828}