[Tabs] Implement Tab#isActivated

- Implement Tab#isActivated() to indicate a tab is active in its
  TabModel. Note that with both an regular and non-incognito TabModel
  this does mean multiple tabs may be activated in one window, but
  only one is active per TabModel.
- Add a test for the behaviors including the edgecase where the tab is
  re-added to the tab model to recover from a pending tab closure.
- Propagate the implementation down to C++ to fulfill the behavior of
  TabInterface for TabAndroid.

Bug: 409366905
Change-Id: I4b2f58706f203e44f3b963f6356b4442a6a88a0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6458860
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Sky Malice <skym@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1447981}