login: Fix clang-3.9 warnings

BUG=none
TEST=daisy-release lumpy-release amd64-generic-asan x86-generic-asan

Change-Id: I1bc1f92d8b69c6a9e5803a2c4d9df1f3a9eae8ed
Reviewed-on: https://chromium-review.googlesource.com/341582
Commit-Ready: Ting-Yuan Huang <laszio@chromium.org>
Tested-by: Ting-Yuan Huang <laszio@chromium.org>
Reviewed-by: Caroline Tice <cmtice@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
diff --git a/login_manager/session_manager_impl_unittest.cc b/login_manager/session_manager_impl_unittest.cc
index 0c91790..8ee6a88 100644
--- a/login_manager/session_manager_impl_unittest.cc
+++ b/login_manager/session_manager_impl_unittest.cc
@@ -889,7 +889,7 @@
 
 TEST_F(SessionManagerImplStaticTest, EmailAddressNonAsciiTest) {
   char invalid[4] = "a@m";
-  invalid[2] = 254;
+  invalid[2] = static_cast<char>(254);
   EXPECT_FALSE(ValidateEmail(invalid));
 }