The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c72833ee18071c957291a378c736835e849d25dd

commit c72833ee18071c957291a378c736835e849d25dd
Author: Chris Blume <cblume@chromium.org>
Date: Thu May 10 22:58:24 2018

Fix aura::WindowPortLocal assumption on embedding

When ParentWindowAllocator changed to default generate a valid
LocalSurfaceId, it broke an implicit assumption that Aura's
WindowPortLocal was relying on.

aura::WindowPortLocal used the default invalid LocalSurfaceId to know it
had not generated a valid ID as the result of embedding external
content.

This patch fixes that by keeping the LocalSurfaceId allocator in a
base::Optional and checking that to determine external content
embedding.

BUG= 840246 

Change-Id: Ie5475e239cc237caaf3a68b149f92812746a7129
Reviewed-on: https://chromium-review.googlesource.com/1046378
Commit-Queue: Chris Blume <cblume@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557717}
[modify] https://crrev.com/c72833ee18071c957291a378c736835e849d25dd/ui/aura/local/window_port_local.cc
[modify] https://crrev.com/c72833ee18071c957291a378c736835e849d25dd/ui/aura/local/window_port_local.h