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When adding a new user, although IsMultiProfileAvailable return true,
MultiUserWindowManagerHelper::GetWindowManager is still null. So add a
check for MultiUserWindowManagerHelper::GetWindowManager. When
MultiUserWindowManagerHelper::GetWindowManager is still null, we don't
need set owner for windows, because multiple user window manager is not
ready yet.

BUG=1047376

Change-Id: Idb527b24e0f7d66714b6f408b08f20e50b6f8c32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032486Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737225}