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Issue:
- When On startup setting is set to "Continue where you left off",
the initial focus is not set after launching the browser.
- Users have to press Tab to focus the window.
- This affects users who rely on screen readers since the window
information is not announced on launch (on UIA mode).

More context:
- The line calling SetInitialFocus was previously in the code but got
removed after a startup focus refactor:
https://chromium-review.googlesource.com/c/chromium/src/+/1313728
- The refactor added the call to RestoreFocus on
BrowserView::OnWidgetActivationChanged if the property
restore_focus_on_activation_ is set to True. This property gets set
during BrowserView::Show, but only in the case of session restore,
Show gets called after OnWidgetActivationChanged so RestoreFocus is
never called.
- The inverted call stack on session restore is because AddRestoredTab
on browser_tabrestore calls Activate before showing the window.

Fix:
- Similar to what we currently have for MacOS, only call Activate
in AddRestoredTab if it is not |from_session_restore|. This way Show
will be called before Activate and thus focus will be set as
expected.
- This change is only made on Windows due to a couple of reasons
1) crbug.com/1102685 is only present on this platform.
2) Making this change on Linux would re-introduce crbug.com/1019048.

Bug: 1102685
Change-Id: I6e62ebad7693eb5d908770364ba7c8d755ccaa70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2284382Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Maria Villarreal <mavill@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#791950}