aura: Fix a tear down crash.

It is possible to receive a capture-lost message while destroying a
WindowTreeHost. Make sure the code works correctly if that happens.

BUG=770670

Change-Id: Idc176e7df5c9b9516264379cd086d441b71c4460
Reviewed-on: https://chromium-review.googlesource.com/696846
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506648}

diff --git a/ui/aura/BUILD.gn b/ui/aura/BUILD.gn
index 0e97f4d..0e1e21f4 100644
--- a/ui/aura/BUILD.gn
+++ b/ui/aura/BUILD.gn

@@ -373,6 +373,7 @@
     "//ui/gfx",
     "//ui/gfx/geometry",
     "//ui/gl:test_support",
+    "//ui/platform_window/stub",
     "//ui/wm",
   ]