[tracing] heap-profiler: Use explicit ConvertableToTraceFormat cast

crrev.com/1427063002 introduced a new overload to the
TRACE_EVENT_API_ADD_METADATA_EVENT api which takes a generic template
as argument. This causes the heap-profiler code in
memory_dump_manager.cc to prefer picking the bool cast operator of
scoped_refptr<DerivedClassWhichExtendsConvertable> rather than
the upcast operator to scoped_refptr<Convertable>.
In practical terms this means that invoking
TRACE_EVENT_API_ADD_METADATA_EVENT("name", scoped_refptr<Derived>)
expands to TRACE_EVENT_API_ADD_METADATA_EVENT("name", true) and does
not invoke the AppendAsTraceFormat() virtual method.
Using the explicit scoped_refptr<Convertable> when invoking the
TRACE_EVENT_API_ADD_METADATA_EVENT to fix the situation.

BUG=542702,524631

Review URL: https://codereview.chromium.org/1429883003

Cr-Commit-Position: refs/heads/master@{#357373}

2 files changed, 7 insertions, 3 deletions

diff --git a/base/trace_event/memory_dump_manager.cc b/base/trace_event/memory_dump_manager.cc
index d939eea..0091442 100644
--- a/base/trace_event/memory_dump_manager.cc
+++ b/base/trace_event/memory_dump_manager.cc
@@ -479,8 +479,9 @@ void MemoryDumpManager::OnTraceLogEnabled() {
     // If heap profiling is enabled, the stack frame deduplicator will be in
     // use. Add a metadata event to write its frames.
     stack_frame_deduplicator = new StackFrameDeduplicator;
-    TRACE_EVENT_API_ADD_METADATA_EVENT("stackFrames", "stackFrames",
-                                       stack_frame_deduplicator);
+    TRACE_EVENT_API_ADD_METADATA_EVENT(
+        "stackFrames", "stackFrames",
+        scoped_refptr<ConvertableToTraceFormat>(stack_frame_deduplicator));
   }
 
   session_state_ = new MemoryDumpSessionState(stack_frame_deduplicator);
diff --git a/base/trace_event/memory_profiler_allocation_context.h b/base/trace_event/memory_profiler_allocation_context.h
index 4e08c50..31c134d 100644
--- a/base/trace_event/memory_profiler_allocation_context.h
+++ b/base/trace_event/memory_profiler_allocation_context.h
@@ -49,7 +49,10 @@ class BASE_EXPORT AllocationStack {
       return;
 
     // Assert that pushes and pops are nested correctly.
-    DCHECK_EQ(frame, stack_.back());
+    // This DCHECK can be hit if some TRACE_EVENT macro is unbalanced
+    // (a TRACE_EVENT_END* call without a corresponding TRACE_EVENT_BEGIN).
+    DCHECK_EQ(frame, stack_.back())
+        << "Encountered an unmatched TRACE_EVENT_END";
 
     stack_.pop_back();
   }