Integration tests for Sync secondary account support

This adds some first integration tests for secondary account support in
Sync: Signing in to a secondary account starts Sync in transport-only
mode, and making the account primary triggers full Sync.

Bug: 871221
Change-Id: I8ce74f3a8a7f819c1fea28347dd6ee4c6c917074
Reviewed-on: https://chromium-review.googlesource.com/1213507
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590247}

chrome/test/BUILD.gn

@@ -5458,6 +5458,7 @@ if (!is_android && !is_fuchsia) {
       "../browser/sync/test/integration/single_client_preferences_sync_test.cc",
       "../browser/sync/test/integration/single_client_printers_sync_test.cc",
       "../browser/sync/test/integration/single_client_search_engines_sync_test.cc",
+      "../browser/sync/test/integration/single_client_secondary_account_sync_test.cc",
       "../browser/sync/test/integration/single_client_sessions_sync_test.cc",
       "../browser/sync/test/integration/single_client_standalone_transport_sync_test.cc",
       "../browser/sync/test/integration/single_client_themes_sync_test.cc",