[platform] Change default platform max thread pool size 4 -> 8

BUG=

Review URL: https://codereview.chromium.org/1782833006

Cr-Commit-Position: refs/heads/master@{#34711}

src/libplatform/default-platform.cc

@@ -29,9 +29,7 @@ bool PumpMessageLoop(v8::Platform* platform, v8::Isolate* isolate) {
   return reinterpret_cast<DefaultPlatform*>(platform)->PumpMessageLoop(isolate);
 }
 
-
-const int DefaultPlatform::kMaxThreadPoolSize = 4;
-
+const int DefaultPlatform::kMaxThreadPoolSize = 8;
 
 DefaultPlatform::DefaultPlatform()
     : initialized_(false), thread_pool_size_(0) {}
@@ -66,7 +64,7 @@ void DefaultPlatform::SetThreadPoolSize(int thread_pool_size) {
   base::LockGuard<base::Mutex> guard(&lock_);
   DCHECK(thread_pool_size >= 0);
   if (thread_pool_size < 1) {
-    thread_pool_size = base::SysInfo::NumberOfProcessors();
+    thread_pool_size = base::SysInfo::NumberOfProcessors() - 1;
   }
   thread_pool_size_ =
       std::max(std::min(thread_pool_size, kMaxThreadPoolSize), 1);