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Since we want to record UMA for session-restored tabs only when they are
initially foreground, we need to track restored-in-foreground tabs. This
CL adds tracking of tabs which are created by session restore and
initially in foreground.

IsTabRestoredInForeground() API added by this CL will be used in
SessionRestorePageLoadMetricsObserver to measure FP, FCP and FMP for
foreground tabs during session restore (http://crrev.com/2930013005/).

started_in_foreground in PageLoadMetricsObserver::OnStart() cannot be
used to determine whether a tab is initially foreground after it is
created by session restore or not. For example, when browser-side
navigation is disabled and a restored tab is created in background,
switching to it before it loads will yield started_in_foreground as
true.

Bug: 731901
Change-Id: I58c6c0330bf6121e78d9d9b9cad72729faeb8885
Reviewed-on: https://chromium-review.googlesource.com/599507Reviewed-by: Zhen Wang <zhenw@chromium.org>
Reviewed-by: Fadi Meawad <fmeawad@chromium.org>
Commit-Queue: Duc Bui <ducbui@google.com>
Cr-Commit-Position: refs/heads/master@{#491821}