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tab_groups_ maps tab model indices to group IDs. This was managed
lazily: entries were created on grouping changes, not as tabs are
added and removed. This is difficult to manage and led to a crash in a
test I was writing.

With this change the entries are created in AddTab() and
AddPinnedTab(). tab_groups_.size() will always match num_tabs_.

Several tests using TabStrip::AddTabAt() directly had to be migrated
to use FakeBaseTabStripController::AddTab(). They relied on the lazy
tab group management and the fact that |tab_groups_.size()| could be
greater than |num_tabs_|.

While I was in there, I migrated all tests to using
FakeBaseTabStripController methods where possible.

Bug: None
Change-Id: I604fd1fca7f0349ad687210fae7ae35452ab1635
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518250Reviewed-by: Taylor Bergquist <tbergquist@chromium.org>
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824228}