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Uses the ExtensionsContainer interface instead of ToolbarActionsBar to
show NTP and DSE warning bubbles. This fixes crashes (DCHECKs) when
using DDG or other default-search-engine extensions.

This also adds the NTP extensions trigger to ExtensionsToolbarContainer
which was used to be able to verify the fix on Linux.

Bug: chromium:985834, chromium:999100
Change-Id: I42b884f2ba7b1978be52daf194abe24f21f453de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1956078
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@{#722660}