signin: Fix bogus assignment in ternary operator

Instead of doing an assignment within one of the expressions, just use the
constant being assigned to. This also fixes a warning produced by GCC:

    ../../chrome/browser/signin/mutable_profile_oauth2_token_service_delegate.cc: In function ‘void {anonymous}::RecordTokenChanged(const string&, const string&)’:
    ../../chrome/browser/signin/mutable_profile_oauth2_token_service_delegate.cc:95:16: warning: operation on ‘transition’ may be undefined [-Wsequence-point]
         transition =
         ~~~~~~~~~~~^
             (new_token ==
             ~~~~~~~~~~~~~

              MutableProfileOAuth2TokenServiceDelegate::kInvalidRefreshToken)
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                 ? TokenStateTransition::kRegularToInvalid
                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                 : transition = TokenStateTransition::kRegularToRegular;
                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Change-Id: I8ea1dbd6ca499f3ba13a36dce41e152dc8263917
Reviewed-on: https://chromium-review.googlesource.com/992323Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#547757}

chrome/browser/signin/mutable_profile_oauth2_token_service_delegate.cc

@@ -96,7 +96,7 @@ void RecordTokenChanged(const std::string& existing_token,
         (new_token ==
          MutableProfileOAuth2TokenServiceDelegate::kInvalidRefreshToken)
             ? TokenStateTransition::kRegularToInvalid
-            : transition = TokenStateTransition::kRegularToRegular;
+            : TokenStateTransition::kRegularToRegular;
   }
   DCHECK_NE(TokenStateTransition::kCount, transition);
   RecordTokenStateTransition(transition);