The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1b8597354a9400f5708d6e1476a51aff1b79ef15

commit 1b8597354a9400f5708d6e1476a51aff1b79ef15
Author: ducbui <ducbui@google.com>
Date: Tue Jun 27 17:06:38 2017

Add signals that mark the start and end of session restore.

Currently, there is no easy way to know whether we are during session restore
or not, so it is necessary to add some signals to do so.

Since the TabLoader lifetime is equal to the session restore, I made TabLoader
to signal the start/end of session restore to the global TabManager.

BUG= 733012 

Review-Url: https://codereview.chromium.org/2935183002
Cr-Commit-Position: refs/heads/master@{#482654}

[modify] https://crrev.com/1b8597354a9400f5708d6e1476a51aff1b79ef15/chrome/browser/resource_coordinator/tab_manager.cc
[modify] https://crrev.com/1b8597354a9400f5708d6e1476a51aff1b79ef15/chrome/browser/resource_coordinator/tab_manager.h
[modify] https://crrev.com/1b8597354a9400f5708d6e1476a51aff1b79ef15/chrome/browser/resource_coordinator/tab_manager_unittest.cc
[modify] https://crrev.com/1b8597354a9400f5708d6e1476a51aff1b79ef15/chrome/browser/sessions/session_restore.cc
[modify] https://crrev.com/1b8597354a9400f5708d6e1476a51aff1b79ef15/chrome/browser/sessions/session_restore.h
[add] https://crrev.com/1b8597354a9400f5708d6e1476a51aff1b79ef15/chrome/browser/sessions/session_restore_observer.h
[add] https://crrev.com/1b8597354a9400f5708d6e1476a51aff1b79ef15/chrome/browser/sessions/session_restore_observer_unittest.cc
[modify] https://crrev.com/1b8597354a9400f5708d6e1476a51aff1b79ef15/chrome/browser/sessions/tab_loader.cc
[modify] https://crrev.com/1b8597354a9400f5708d6e1476a51aff1b79ef15/chrome/test/BUILD.gn

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bf1f56937292f77b1000abaac18658e3bf5ada50

commit bf1f56937292f77b1000abaac18658e3bf5ada50
Author: Duc Bui <ducbui@google.com>
Date: Wed Jul 19 14:42:07 2017

[TabManager] Fix Session restore started signal

Move the Start signal from TabLoader constructor to earlier points.

TabLoader is created after navigation started therefore the 
PageLoadMetricsObserver is created before the current session 
restore started signal (only when browser side navigation is enabled).

This CL moves the signal earlier such that the Session restore started
signal is always invoked before any PageLoadMetricsObserver creation.
This CL also adds support for RestoreForeignSession.

Design doc:
https://docs.google.com/a/google.com/document/d/1sS1WO2rp6M8g1Uco1GMsyqr5wsuJjQzA9RTgrNZQTxA/edit?usp=sharing

BUG: chromium:733012 

Change-Id: I23373e802c1a37226e4d48395b6954c630ff0c5f
Reviewed-on: https://chromium-review.googlesource.com/564343
Reviewed-by: Zhen Wang <zhenw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Duc Bui <ducbui@google.com>
Cr-Commit-Position: refs/heads/master@{#487853}
[modify] https://crrev.com/bf1f56937292f77b1000abaac18658e3bf5ada50/chrome/browser/resource_coordinator/tab_manager_unittest.cc
[modify] https://crrev.com/bf1f56937292f77b1000abaac18658e3bf5ada50/chrome/browser/sessions/session_restore.cc
[modify] https://crrev.com/bf1f56937292f77b1000abaac18658e3bf5ada50/chrome/browser/sessions/session_restore.h
[modify] https://crrev.com/bf1f56937292f77b1000abaac18658e3bf5ada50/chrome/browser/sessions/session_restore_observer.h
[modify] https://crrev.com/bf1f56937292f77b1000abaac18658e3bf5ada50/chrome/browser/sessions/session_restore_observer_unittest.cc
[modify] https://crrev.com/bf1f56937292f77b1000abaac18658e3bf5ada50/chrome/browser/sessions/tab_loader.cc