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This adds an integration test, testing changing element conditions.

This CL also fixes several bugs that were discovered during testing:
- Fixed issue where ElementConditions were not correctly added and
cleared.
- Constructing a new instance of AssistantHeaderCoordinator no longer
has a side effect in AssistantHeaderModel.
- Fixed a possible crash when showing a new trigger script while another
was in the process of being hidden (crash comes down to timing). The fix
is to always create a new BottomSheetContent, instead of re-populating
the old one.

Bug: b/171776026
Change-Id: I7eac6673100ab21d459ec7f1ed200b2be9a44ff5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2524532
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Marian Fechete <marianfe@google.com>
Cr-Commit-Position: refs/heads/master@{#826379}