[FedCM] Parse config field for supporting multiple accounts

Tentatively called "supports_multiple_accounts" in the root of the
config dictionary.

R=npm@chromium.org

Bug: 1498628
Change-Id: If96e99d1b2dc0c320b94930f441eb2b2f2b5adbd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5002071
Auto-Submit: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Nicolás Peña <npm@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1219610}

content/browser/webid/idp_network_request_manager.cc

@@ -69,6 +69,7 @@ constexpr char kIdAssertionEndpoint[] = "id_assertion_endpoint";
 constexpr char kClientMetadataEndpointKey[] = "client_metadata_endpoint";
 constexpr char kMetricsEndpoint[] = "metrics_endpoint";
 constexpr char kRevocationEndpoint[] = "revocation_endpoint";
+constexpr char kSupportsAddAccountKey[] = "supports_add_account";
 
 // Shared between the well-known files and config files
 constexpr char kAccountsEndpointKey[] = "accounts_endpoint";
@@ -532,6 +533,8 @@ void OnConfigParsed(const GURL& provider,
   }
   idp_metadata.idp_login_url =
       ExtractEndpoint(provider, response, kLoginUrlKey);
+  idp_metadata.supports_add_account =
+      response.FindBool(kSupportsAddAccountKey).value_or(false);
   std::move(callback).Run({ParseStatus::kSuccess, fetch_status.response_code},
                           endpoints, std::move(idp_metadata));
 }

content/browser/webid/idp_network_request_manager_unittest.cc

@@ -838,6 +838,35 @@ TEST_F(IdpNetworkRequestManagerTest, ParseConfigBrandingMinSize) {
   }
 }
 
+TEST_F(IdpNetworkRequestManagerTest, ParseConfigSupportsAddAccount) {
+  const char test_json[] = R"({
+  "supports_add_account": true
+  })";
+
+  FetchStatus fetch_status;
+  IdentityProviderMetadata idp_metadata;
+  std::tie(fetch_status, idp_metadata) =
+      SendConfigRequestAndWaitForResponse(test_json);
+
+  EXPECT_EQ(ParseStatus::kSuccess, fetch_status.parse_status);
+  EXPECT_EQ(net::HTTP_OK, fetch_status.response_code);
+  EXPECT_EQ(true, idp_metadata.supports_add_account);
+}
+
+TEST_F(IdpNetworkRequestManagerTest, ParseConfigSupportsAddAccountMissing) {
+  const char test_json[] = R"({
+  })";
+
+  FetchStatus fetch_status;
+  IdentityProviderMetadata idp_metadata;
+  std::tie(fetch_status, idp_metadata) =
+      SendConfigRequestAndWaitForResponse(test_json);
+
+  EXPECT_EQ(ParseStatus::kSuccess, fetch_status.parse_status);
+  EXPECT_EQ(net::HTTP_OK, fetch_status.response_code);
+  EXPECT_EQ(false, idp_metadata.supports_add_account);
+}
+
 // Tests that we send the correct origin for account requests.
 TEST_F(IdpNetworkRequestManagerTest, AccountRequestOrigin) {
   bool called = false;

content/public/browser/identity_request_dialog_controller.h

@@ -47,6 +47,8 @@ struct CONTENT_EXPORT IdentityProviderMetadata {
   // IdentityProviderMetadata so that the UI code can pass it along when an
   // Account is selected by the user.
   GURL config_url;
+  // Whether this IdP supports signing in to additional accounts.
+  bool supports_add_account{false};
 };
 
 struct CONTENT_EXPORT IdentityProviderData {