[Tracing] Add hook to PartitionAlloc for heap profiling

base/trace_event/memory_profiler_allocation_context.cc

Index: base/trace_event/memory_profiler_allocation_context.cc
diff --git a/base/trace_event/memory_profiler_allocation_context.cc b/base/trace_event/memory_profiler_allocation_context.cc
index b82e2109845b0494c82ee7d689aefe5f7a7d960b..d6444abe47f9c7f8c567d2e5f6aefd45353252ef 100644
--- a/base/trace_event/memory_profiler_allocation_context.cc
+++ b/base/trace_event/memory_profiler_allocation_context.cc
@@ -15,11 +15,32 @@ subtle::Atomic32 AllocationContextTracker::capture_enabled_ = 0;
 
 namespace {
 ThreadLocalStorage::StaticSlot g_tls_alloc_ctx_tracker = TLS_INITIALIZER;
+
+// Returns a pointer past the end of the fixed-size array |array| of |T| of
+// length |N|, identical to C++11 |std::end|.
+template <typename T, int N>
+T* End(T(&array)[N]) {
+  return array + N;
 }
 
+}  // namespace
+
 AllocationStack::AllocationStack() {}
 AllocationStack::~AllocationStack() {}
 
+AllocationContext AllocationContext::Empty() {
+  AllocationContext context;
+
+  for (StackFrame* i = context.backtrace.frames;
+       i != End(context.backtrace.frames); i++)

[Primiano Tucci (use gerrit), 2015/10/12 16:36:45]

You can make this simpler, shorter and more readable without using the End()
template magic:

  for (size_t i = 0; i < arraysize(context.backtrace.frames); ++i)
    context.backtrace.frames[i] = nullptr

Also "i" for a non-numeric type is a very bad naming choice.
Furthermore this for loop would require extra braces around the *i = nullptr as
the for spawns across multiple lines.

[Ruud van Asseldonk, 2015/10/13 10:42:18]

Done.

+    *i = nullptr;
+
+  for (auto* i = context.fields; i != End(context.fields); i++)

[Primiano Tucci (use gerrit), 2015/10/12 16:36:45]

ditto

[Ruud van Asseldonk, 2015/10/13 10:42:18]

Done.

+    i->first = nullptr;
+
+  return context;
+}
+
 // This function is added to the TLS slot to clean up the instance when the
 // thread exits.
 void DestructAllocationContextTracker(void* alloc_ctx_tracker) {
@@ -81,13 +102,6 @@ void AllocationContextTracker::UnsetContextField(const char* key) {
   tracker->context_.erase(key);
 }
 
-// Returns a pointer past the end of the fixed-size array |array| of |T| of
-// length |N|, identical to C++11 |std::end|.
-template <typename T, int N>
-T* End(T(&array)[N]) {
-  return array + N;
-}
-
 // static
 AllocationContext AllocationContextTracker::GetContextSnapshot() {
   AllocationContextTracker* tracker = GetThreadLocalTracker();