[tracing] Add native allocation tracing mode.

base/trace_event/malloc_dump_provider.cc

Index: base/trace_event/malloc_dump_provider.cc
diff --git a/base/trace_event/malloc_dump_provider.cc b/base/trace_event/malloc_dump_provider.cc
index 17e5c1e62074f95720442ee3b49a944607b490bf..38fb725b6f758339627703fa59257f95ad97e668 100644
--- a/base/trace_event/malloc_dump_provider.cc
+++ b/base/trace_event/malloc_dump_provider.cc
@@ -236,12 +236,12 @@ void MallocDumpProvider::InsertAllocation(void* address, size_t size) {
   auto tracker = AllocationContextTracker::GetInstanceForCurrentThread();
   if (!tracker)
     return;
-  AllocationContext context = tracker->GetContextSnapshot();
 
   AutoLock lock(allocation_register_lock_);
   if (!allocation_register_)
     return;
 
+  AllocationContext context = tracker->GetContextSnapshot();

[Primiano Tucci (use gerrit), 2016/04/07 15:51:57]

why moving this?
The point of this being before the lock is that you don't need to stall all the
threads that are doing a malloc while you do the stack unwinding.
If you are worried about hitting the NOTREACHED() in the NOT_ENABLED case, as I
told above, remove the NOTREACHED() and let it happen.
I'd not be too much obsessed about dealing with the corner cases on tracing
enabled (as long as we don't return garbage) and prefer performances.

   allocation_register_->Insert(address, size, context);
 }