[compiler-dispatcher] Allow limiting the number of threads

Add a flag for controlling the maximum number of threads usable by the
LazyCompileDispatcher.

Change-Id: I87fc39d337a00cff0c3d152392630f48da050fff
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3312482
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78214}

src/compiler-dispatcher/lazy-compile-dispatcher.cc

@@ -35,8 +35,11 @@ class LazyCompileDispatcher::JobTask : public v8::JobTask {
   }
 
   size_t GetMaxConcurrency(size_t worker_count) const final {
-    return lazy_compile_dispatcher_->num_jobs_for_background_.load(
+    size_t n = lazy_compile_dispatcher_->num_jobs_for_background_.load(
         std::memory_order_relaxed);
+    if (FLAG_lazy_compile_dispatcher_max_threads == 0) return n;
+    return std::min(
+        n, static_cast<size_t>(FLAG_lazy_compile_dispatcher_max_threads));
   }
 
  private:

src/flags/flag-definitions.h

@@ -1578,6 +1578,8 @@ DEFINE_BOOL(cache_prototype_transitions, true, "cache prototype transitions")
 
 // lazy-compile-dispatcher.cc
 DEFINE_BOOL(lazy_compile_dispatcher, false, "enable compiler dispatcher")
+DEFINE_UINT(lazy_compile_dispatcher_max_threads, 0,
+            "max threads for compiler dispatcher (0 for unbounded)")
 DEFINE_BOOL(trace_compiler_dispatcher, false,
             "trace compiler dispatcher activity")
 DEFINE_BOOL(