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Replace old API with new API. This CL is a refactor and has no
intended behavior change.

In the process, Introduce BrowserWindow::OnTabDetached()

When closing a tab with old API, expected order of calls are..

1. Browser::TabDetached
2. BrowserView::TabDetached -> this resets contents_web_view_.
3. Browser::ActiveTabChanged() -> This will call
BrowserWindow(BrowserView)::OnActiveTabChange() : Set new contents to
contents_web_view_

But, if we use the new observer API it would be

1. Browser::OnTabStripModelChanged()
    * handles things Browser::TabDetached() used to do.
    * handles things Browser::ActiveTabChanged() used to
2. BrowserView::OnTabStripModelChanged()
    * handles BrowserView:TabDetachedAt() used to do.

As a result, contents_web_view_ is cleared out with null
contents.

Repro step and the result would be,
1. Open browser
2. Create new tab
3. Close active tab.
-> Then new active tab's contents won't be visible.

Therefore, Introduce BrowserWindow::OnTabDetached() and
let the Browser control the flow.

Change-Id: I88c08f05d365f83146602197374299d4b4fdb14c
Reviewed-on: https://chromium-review.googlesource.com/c/1205975
Commit-Queue: Sang Woo Ko <sangwoo108@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601844}