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Previously, the notifications were sent during the tab-close-loop [if multiple
tabs were being closed]. This made reentrancy particularly dangerous.

The logic in Browser::TabDetachedAt and Browser::TabInsertedAt relied on the
|index| parameter to determine whether the index of the active tab has changed
[for which there is no notification]. This CL changes those methods to always
call SessionService::SetSelectedTabInWindow, and modifies
SessionService::SetSelectedTabInWindow to be idempotent.

Bug: 842194
Change-Id: I2fc54749e495ab9c925bdba89ef857c4ea3537d9
Reviewed-on: https://chromium-review.googlesource.com/1055732Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558530}