Attempt migrations once (#528)

Author: camden-king

GoogleSignIn/Sources/GIDAuthStateMigration.m

@@ -107,10 +107,6 @@ - (void)performDataProtectedMigrationIfNeeded {
   if (authSession) {
     NSError *err;
     [self.keychainStore saveAuthSession:authSession error:&err];
-    // If we're unable to save to the keychain, return without marking migration performed.
-    if (err) {
-      return;
-    };
     [keychainStoreLegacy removeAuthSessionWithError:nil];
   }
 
@@ -138,10 +134,6 @@ - (void)performGIDMigrationIfNeededWithTokenURL:(NSURL *)tokenURL
   if (authSession) {
     NSError *err;
     [self.keychainStore saveAuthSession:authSession error:&err];
-    // If we're unable to save to the keychain, return without marking migration performed.
-    if (err) {
-      return;
-    };
   }
 
   // Mark the migration check as having been performed.

GoogleSignIn/Tests/Unit/GIDAuthStateMigrationTest.m

@@ -181,7 +181,7 @@ - (void)testMigrateIfNeeded_KeychainFailure_DataProtectedMigration {
                             callbackPath:kCallbackPath
                             keychainName:kKeychainName
                           isFreshInstall:NO];
-  XCTAssertNotNil([_realLegacyGTMKeychainStore retrieveAuthSessionWithError:nil]);
+  XCTAssertNil([_realLegacyGTMKeychainStore retrieveAuthSessionWithError:nil]);
 }
 
 #else