tracing: make GetCategoryGroupEnabled thread-safe

Change-Id: I29795e6df81f8ec719bdf62e5625cc06717861a5
Reviewed-on: https://chromium-review.googlesource.com/1183960Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#55318}

src/libplatform/tracing/tracing-controller.cc

@@ -190,17 +190,21 @@ const uint8_t* TracingController::GetCategoryGroupEnabledInternal(
   DCHECK(!strchr(category_group, '"'));
 
   // The g_category_groups is append only, avoid using a lock for the fast path.
-  size_t current_category_index = v8::base::Acquire_Load(&g_category_index);
+  size_t category_index = base::Acquire_Load(&g_category_index);
 
   // Search for pre-existing category group.
-  for (size_t i = 0; i < current_category_index; ++i) {
+  for (size_t i = 0; i < category_index; ++i) {
     if (strcmp(g_category_groups[i], category_group) == 0) {
       return &g_category_group_enabled[i];
     }
   }
 
+  // Slow path. Grab the lock.
+  base::LockGuard<base::Mutex> lock(mutex_.get());
+
+  // Check the list again with lock in hand.
   unsigned char* category_group_enabled = nullptr;
-  size_t category_index = base::Acquire_Load(&g_category_index);
+  category_index = base::Acquire_Load(&g_category_index);
   for (size_t i = 0; i < category_index; ++i) {
     if (strcmp(g_category_groups[i], category_group) == 0) {
       return &g_category_group_enabled[i];