Add TabContextObserver

TabContextObserver is used to signal changes to the tab model.
It has applications where we want to prefetch the tab close
suggestions after the tab model changes so they are available
for the UI thread (and the UI thread doesn't have to block on them).

Bug: 1005573
Change-Id: I91ea9367a90301d6bc6dc9a00c141c59a574db5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1811935


Commit-Queue: David Maunder <davidjm@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700294}