[Counters] Only create TLS key for WorkerThreadRuntimeCallStats if used.

BUG=chromium:919394

Change-Id: I99d0b557f8d52c87c4422cd971a43211e5d5e849
Reviewed-on: https://chromium-review.googlesource.com/c/1425198Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58964}

src/counters.cc

@@ -531,11 +531,16 @@ void RuntimeCallStats::Dump(v8::tracing::TracedValue* value) {
   in_use_ = false;
 }
 
-WorkerThreadRuntimeCallStats::WorkerThreadRuntimeCallStats()
-    : tls_key_(base::Thread::CreateThreadLocalKey()) {}
+WorkerThreadRuntimeCallStats::WorkerThreadRuntimeCallStats() {}
 
 WorkerThreadRuntimeCallStats::~WorkerThreadRuntimeCallStats() {
-  base::Thread::DeleteThreadLocalKey(tls_key_);
+  if (tls_key_) base::Thread::DeleteThreadLocalKey(*tls_key_);
+}
+
+base::Thread::LocalStorageKey WorkerThreadRuntimeCallStats::GetKey() {
+  DCHECK(FLAG_runtime_stats);
+  if (!tls_key_) tls_key_ = base::Thread::CreateThreadLocalKey();
+  return *tls_key_;
 }
 
 RuntimeCallStats* WorkerThreadRuntimeCallStats::NewTable() {

src/counters.h

@@ -8,6 +8,7 @@
 #include "include/v8.h"
 #include "src/allocation.h"
 #include "src/base/atomic-utils.h"
+#include "src/base/optional.h"
 #include "src/base/platform/elapsed-timer.h"
 #include "src/base/platform/time.h"
 #include "src/globals.h"
@@ -1101,7 +1102,7 @@ class WorkerThreadRuntimeCallStats final {
   ~WorkerThreadRuntimeCallStats();
 
   // Returns the TLS key associated with this WorkerThreadRuntimeCallStats.
-  base::Thread::LocalStorageKey GetKey() const { return tls_key_; }
+  base::Thread::LocalStorageKey GetKey();
 
   // Returns a new worker thread runtime call stats table managed by this
   // WorkerThreadRuntimeCallStats.
@@ -1113,7 +1114,7 @@ class WorkerThreadRuntimeCallStats final {
  private:
   base::Mutex mutex_;
   std::vector<std::unique_ptr<RuntimeCallStats>> tables_;
-  base::Thread::LocalStorageKey tls_key_;
+  base::Optional<base::Thread::LocalStorageKey> tls_key_;
 };
 
 // Creating a WorkerThreadRuntimeCallStatsScope will provide a thread-local