Fix tabs staying hovered when they are pinned with SplitTabStrip on.

CompoundTabContainer was removing the pinning tab from its source container and then adding it to itself to animate the pin. This was causing RootView to lose track of the tab, which it was tracking as `mouse_move_handler_`, which meant the tab never received an OnMouseExited event and it stayed in its hover state.

Now, the tab is directly transferred to the CompoundTabContainer, which RootView can follow, so the tab receives its OnMouseExited event as desired.

IMO this is also a small readability improvement to the TabContainer API, so that's a nice side bonus.

Bug: 1379248
Change-Id: Idc2b2e6dc1adf531fbe5200ac670238f09c4d15b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4163621
Reviewed-by: Peter Boström <pbos@chromium.org>
Auto-Submit: Taylor Bergquist <tbergquist@chromium.org>
Commit-Queue: Taylor Bergquist <tbergquist@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1095914}