Remove flakiness from Cast Core gRPC tests.

Due to thread timing, a non-null reactor may be received on the server
side, which is then followed by a proper null-reactor callback. The fix
adjusts for such a behavior.

Bug: 374999320
Change-Id: Ic3b95dddcf8c230da0f69f0672f3065eec8ec51c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5958328
Reviewed-by: Antonio Rivera <antoniori@google.com>
Commit-Queue: Vigen Issahhanjan <vigeni@google.com>
Cr-Commit-Position: refs/heads/main@{#1372974}

chromecast/cast_core/grpc/grpc_server_streaming_test.cc

@@ -230,16 +230,8 @@ TEST_F(GrpcServerStreamingTest, ServerStreamingCallIsCancelledByClient) {
   task_environment_.RunUntilIdle();
 }
 
-// Flaky on Android and Linux, see https://crbug.com/374999320.
-#if BUILDFLAG(IS_ANDROID) || BUILDFLAG(IS_LINUX)
-#define MAYBE_ServerStreamingCallIsCancelledByClientInActiveReactor \
-  DISABLED_ServerStreamingCallIsCancelledByClientInActiveReactor
-#else
-#define MAYBE_ServerStreamingCallIsCancelledByClientInActiveReactor \
-  ServerStreamingCallIsCancelledByClientInActiveReactor
-#endif
 TEST_F(GrpcServerStreamingTest,
-       MAYBE_ServerStreamingCallIsCancelledByClientInActiveReactor) {
+       ServerStreamingCallIsCancelledByClientInActiveReactor) {
   GrpcServer server;
 
   base::WaitableEvent server_request_received;
@@ -275,7 +267,12 @@ TEST_F(GrpcServerStreamingTest,
   cancelled_reactor->SetWritesAvailableCallback(base::BindLambdaForTesting(
       [&](grpc::Status status,
           ServerStreamingServiceHandler::StreamingCall::Reactor* reactor) {
-        ASSERT_THAT(reactor, IsNull());
+        if (reactor) {
+          // Timing between threds may result in server reactor still receiving
+          // the empty TestResponse from below. Ignore it and continue waiting
+          // for the null reactor.
+          return;
+        }
         ASSERT_THAT(status, StatusIs(grpc::StatusCode::ABORTED));
         reactor_aborted.Signal();
       }));