Fix sharing preference between Linux and ChromeOS on Linux

Build of ChromeOS running on Linux may share preference with
the Linux build of Chrome. Ensure that the migration state is
set to MIGRATION_NOT_STARTED on ChromeOS.

Bug: 879968
Change-Id: Ifff879198a51555c6a88c5c2a5712160745c64f6
Reviewed-on: https://chromium-review.googlesource.com/1202103Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588400}

components/signin/core/browser/account_tracker_service.cc

@@ -571,7 +571,11 @@ void AccountTrackerService::LoadFromPrefs() {
       }
     }
   } else {
-    DCHECK_EQ(GetMigrationState(), MIGRATION_NOT_STARTED);
+    // ChromeOS running on Linux and Linux share the preferences, so the
+    // migration may have been performed on Linux. Reset the migration
+    // state to ensure that the same code path is used whether ChromeOS
+    // is running on Linux on a dev build or on real ChromeOS device.
+    SetMigrationState(MIGRATION_NOT_STARTED);
   }
 
   DCHECK(GetMigrationState() != MIGRATION_DONE || IsMigrationDone());