[TraceLog] Fix uninitialized members

Also renamed the types to "Functions" instead of "Callbacks" as the
latter is confusing with a properly bound base::Callback where the
former clearly denotates a function pointer.

This lack of initialization resulted in an access violation instead of
using the non-overridden function when tracing is called in contexts
where the perfetto override isn't in place yet as part of
https://chromium-review.googlesource.com/c/chromium/src/+/1680893

We were simply lucky to get away with until now.

R=eseckler@chromium.org

Bug: 899897
Change-Id: I815fe5197888c32200cdf5166a0be4fbf79d355a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1721071
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681769}

base/trace_event/trace_log.cc

@@ -331,12 +331,12 @@ void TraceLog::ThreadLocalEventBuffer::FlushWhileLocked() {
 }
 
 void TraceLog::SetAddTraceEventOverrides(
-    const AddTraceEventOverrideCallback& add_event_override,
-    const OnFlushCallback& on_flush_callback,
-    const UpdateDurationCallback& update_duration_callback) {
+    const AddTraceEventOverrideFunction& add_event_override,
+    const OnFlushFunction& on_flush_override,
+    const UpdateDurationFunction& update_duration_override) {
   add_trace_event_override_.store(add_event_override);
-  on_flush_callback_.store(on_flush_callback);
-  update_duration_callback_.store(update_duration_callback);
+  on_flush_override_.store(on_flush_override);
+  update_duration_override_.store(update_duration_override);
 }
 
 struct TraceLog::RegisteredAsyncObserver {
@@ -1033,9 +1033,9 @@ void TraceLog::FlushCurrentThread(int generation, bool discard_events) {
   // This will flush the thread local buffer.
   delete thread_local_event_buffer_.Get();
 
-  auto on_flush_callback = on_flush_callback_.load(std::memory_order_relaxed);
-  if (on_flush_callback) {
-    on_flush_callback();
+  auto on_flush_override = on_flush_override_.load(std::memory_order_relaxed);
+  if (on_flush_override) {
+    on_flush_override();
   }
 
   // Scheduler uses TRACE_EVENT macros when posting a task, which can lead
@@ -1432,10 +1432,10 @@ void TraceLog::UpdateTraceEventDurationExplicit(
 #endif  // OS_WIN
 
   if (category_group_enabled_local & TraceCategory::ENABLED_FOR_RECORDING) {
-    auto update_duration_callback =
-        update_duration_callback_.load(std::memory_order_relaxed);
-    if (update_duration_callback) {
-      update_duration_callback(handle, now, thread_now, thread_instruction_now);
+    auto update_duration_override =
+        update_duration_override_.load(std::memory_order_relaxed);
+    if (update_duration_override) {
+      update_duration_override(handle, now, thread_now, thread_instruction_now);
       return;
     }
   }

base/trace_event/trace_log.h

@@ -196,11 +196,11 @@ class BASE_EXPORT TraceLog : public MemoryDumpProvider {
   // Cancels tracing and discards collected data.
   void CancelTracing(const OutputCallback& cb);
 
-  using AddTraceEventOverrideCallback = void (*)(TraceEvent*,
+  using AddTraceEventOverrideFunction = void (*)(TraceEvent*,
                                                  bool thread_will_flush,
                                                  TraceEventHandle* handle);
-  using OnFlushCallback = void (*)();
-  using UpdateDurationCallback =
+  using OnFlushFunction = void (*)();
+  using UpdateDurationFunction =
       void (*)(TraceEventHandle handle,
                const TimeTicks& now,
                const ThreadTicks& thread_now,
@@ -209,9 +209,9 @@ class BASE_EXPORT TraceLog : public MemoryDumpProvider {
   // finished, i.e. must be callable until OutputCallback is called with
   // has_more_events==false.
   void SetAddTraceEventOverrides(
-      const AddTraceEventOverrideCallback& add_event_override,
-      const OnFlushCallback& on_flush_callback,
-      const UpdateDurationCallback& update_duration_callback);
+      const AddTraceEventOverrideFunction& add_event_override,
+      const OnFlushFunction& on_flush_callback,
+      const UpdateDurationFunction& update_duration_callback);
 
   // Called by TRACE_EVENT* macros, don't call this directly.
   // The name parameter is a category group for example:
@@ -541,9 +541,9 @@ class BASE_EXPORT TraceLog : public MemoryDumpProvider {
   MetadataFilterPredicate metadata_filter_predicate_;
   subtle::AtomicWord generation_;
   bool use_worker_thread_;
-  std::atomic<AddTraceEventOverrideCallback> add_trace_event_override_;
-  std::atomic<OnFlushCallback> on_flush_callback_;
-  std::atomic<UpdateDurationCallback> update_duration_callback_;
+  std::atomic<AddTraceEventOverrideFunction> add_trace_event_override_{nullptr};
+  std::atomic<OnFlushFunction> on_flush_override_{nullptr};
+  std::atomic<UpdateDurationFunction> update_duration_override_{nullptr};
 
   FilterFactoryForTesting filter_factory_for_testing_;