[fuzzing] Allow flow control fuzzer to fuzz across experiments (#33241)

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---------

Co-authored-by: ctiller <ctiller@users.noreply.github.com>

test/core/transport/chttp2/BUILD

@@ -50,12 +50,14 @@ grpc_proto_fuzzer(
     corpus = "flow_control_fuzzer_corpus",
     language = "C++",
     proto = "flow_control_fuzzer.proto",
+    proto_deps = ["//test/core/util:fuzz_config_vars_proto"],
     tags = ["no_windows"],
     uses_event_engine = False,
     uses_polling = False,
     deps = [
         "//:gpr",
         "//src/core:chttp2_flow_control",
+        "//test/core/util:fuzz_config_vars",
     ],
 )
 

test/core/transport/chttp2/flow_control_fuzzer.cc

@@ -37,6 +37,7 @@
 #include <grpc/support/time.h>
 
 #include "src/core/ext/transport/chttp2/transport/flow_control.h"
+#include "src/core/lib/experiments/config.h"
 #include "src/core/lib/gpr/useful.h"
 #include "src/core/lib/gprpp/time.h"
 #include "src/core/lib/iomgr/exec_ctx.h"
@@ -44,6 +45,7 @@
 #include "src/core/lib/transport/bdp_estimator.h"
 #include "src/libfuzzer/libfuzzer_macro.h"
 #include "test/core/transport/chttp2/flow_control_fuzzer.pb.h"
+#include "test/core/util/fuzz_config_vars.h"
 
 // IWYU pragma: no_include <google/protobuf/repeated_ptr_field.h>
 
@@ -442,6 +444,8 @@ void FlowControlFuzzer::AssertAnnouncedOverInitialWindowSizeCorrect() const {
 }  // namespace grpc_core
 
 DEFINE_PROTO_FUZZER(const flow_control_fuzzer::Msg& msg) {
+  grpc_core::ApplyFuzzConfigVars(msg.config_vars());
+  grpc_core::TestOnlyReloadExperimentsFromConfigVariables();
   grpc_core::chttp2::InitGlobals();
   grpc_core::chttp2::FlowControlFuzzer fuzzer(msg.enable_bdp());
   for (const auto& action : msg.actions()) {

test/core/transport/chttp2/flow_control_fuzzer.proto

@@ -16,6 +16,8 @@ syntax = "proto3";
 
 package flow_control_fuzzer;
 
+import "test/core/util/fuzz_config_vars.proto";
+
 message Empty {};
 
 message StreamWrite {
@@ -44,4 +46,5 @@ message Action {
 message Msg {
     bool enable_bdp = 1;
     repeated Action actions = 2;
+    grpc.testing.FuzzConfigVars config_vars = 3;
 }