GitLab

This CL:
- Removes AssistantCardElement one-off logic from AssistantResponse so
  that it will scale to new AssisantUiElement types.
- Moves private classes into .cc to clean up .h

Bug: b:145003512
Change-Id: If6a9c79a7a6a49c10d903762ac9c1d82a8267c30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2006137
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733011}