[tracing] Don't pop context from an empty stack.

base/trace_event/heap_profiler_allocation_context_tracker.cc

Index: base/trace_event/heap_profiler_allocation_context_tracker.cc
diff --git a/base/trace_event/heap_profiler_allocation_context_tracker.cc b/base/trace_event/heap_profiler_allocation_context_tracker.cc
index e192bc833fd950507a5756387b84b5fc92e827ed..46d30acffa3a9f2a85a678ccf0af3ef899465844 100644
--- a/base/trace_event/heap_profiler_allocation_context_tracker.cc
+++ b/base/trace_event/heap_profiler_allocation_context_tracker.cc
@@ -113,6 +113,12 @@ void AllocationContextTracker::PushCurrentTaskContext(const char* context) {
 }
 
 void AllocationContextTracker::PopCurrentTaskContext(const char* context) {
+  // Guard for stack underflow. If tracing was started with a TRACE_EVENT in
+  // scope, the context was never pushed, so it is possible that pop is called
+  // on an empty stack.
+  if (task_contexts_.empty())
+    return;
+
   DCHECK_EQ(context, task_contexts_.back())

[Primiano Tucci (use gerrit), 2016/04/29 12:02:36]

ok following this logic (that is correcT) we should also remove this DCHECK then
(and the |context| argument alltogether)

[ssid, 2016/04/29 12:10:44]

This check is still valid because the contexts cannot be misordered. Either the
context can go missing which means the stack will be empty or the context should
match the top.

This is unlike trace event because once the enabled flag is set it's never
unset. while tracing can be disabled and events can miss ordering in pseudo
stack.

       << "Encountered an unmatched context end";
   task_contexts_.pop_back();