Add unit tests for Signin Header Helpers.

components/signin/core/browser/BUILD.gn

Index: components/signin/core/browser/BUILD.gn
diff --git a/components/signin/core/browser/BUILD.gn b/components/signin/core/browser/BUILD.gn
index c5f5f2323b70797a066256f90148fcfbd2d96657..3f808e19faf35f2ecae38b5c65dd00fb0fc6cbf8 100644
--- a/components/signin/core/browser/BUILD.gn
+++ b/components/signin/core/browser/BUILD.gn
@@ -152,6 +152,7 @@ source_set("unit_tests") {
     "gaia_cookie_manager_service_unittest.cc",
     "refresh_token_annotation_request_unittest.cc",
     "signin_error_controller_unittest.cc",
+    "signin_header_helper_unittest.cc",
     "signin_investigator_unittest.cc",
     "signin_status_metrics_provider_unittest.cc",
     "webdata/token_service_table_unittest.cc",
@@ -159,7 +160,9 @@ source_set("unit_tests") {
 
   deps = [
     ":test_support",
+    "//components/content_settings/core/browser",
     "//components/os_crypt:test_support",
+    "//components/pref_registry:test_support",
     "//components/signin/core/common",
     "//components/syncable_prefs:test_support",
     "//testing/gmock",