[test] Enable passing the thread-pool size to d8

This will enable some fuzzers to alter the thread-pool size.

Bug: v8:7455
Change-Id: Ic9c9600cdb3dc50e860dbda8432a23bb20f1dd44
Reviewed-on: https://chromium-review.googlesource.com/924273Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51335}

src/d8.cc

@@ -2898,6 +2898,9 @@ bool Shell::SetOptions(int argc, char* argv[]) {
     } else if (strcmp(argv[i], "--quiet-load") == 0) {
       options.quiet_load = true;
       argv[i] = nullptr;
+    } else if (strncmp(argv[i], "--thread-pool-size=", 19) == 0) {
+      options.thread_pool_size = atoi(argv[i] + 19);
+      argv[i] = nullptr;
     }
   }
 
@@ -3326,8 +3329,8 @@ int Shell::Main(int argc, char* argv[]) {
 
   platform::tracing::TracingController* tracing_controller = tracing.get();
   g_platform = v8::platform::NewDefaultPlatform(
-      0, v8::platform::IdleTaskSupport::kEnabled, in_process_stack_dumping,
-      std::move(tracing));
+      options.thread_pool_size, v8::platform::IdleTaskSupport::kEnabled,
+      in_process_stack_dumping, std::move(tracing));
   if (i::FLAG_verify_predictable) {
     g_platform.reset(new PredictablePlatform(std::move(g_platform)));
   }

src/d8.h

@@ -351,6 +351,7 @@ class ShellOptions {
   int read_from_tcp_port;
   bool enable_os_system = false;
   bool quiet_load = false;
+  int thread_pool_size = 0;
 };
 
 class Shell : public i::AllStatic {