Fix crash in ExtensionUninstallDialogViews::Show

Fixes regression in r728281. The code started assuming that a
BrowserActionsContainer always existed, this is not true for windows
without a tab strip.

Bug: chromium:1042419
Change-Id: Ia69257972ebe08d58a878722a42821730ebcc7b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2003220
Commit-Queue: Peter Boström <pbos@chromium.org>
Commit-Queue: Caroline Rising <corising@chromium.org>
Auto-Submit: Peter Boström <pbos@chromium.org>
Reviewed-by: Caroline Rising <corising@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732258}