tracing: Simplify TraceEventMemoryOverhead, use an enum insted of a map

base/trace_event/trace_buffer.cc

Index: base/trace_event/trace_buffer.cc
diff --git a/base/trace_event/trace_buffer.cc b/base/trace_event/trace_buffer.cc
index e26e9fd28fa806cfedcc00093dd5b336f8486dc0..0a5770c3dbf0058715b64222bf1ae4db4ae1abf3 100644
--- a/base/trace_event/trace_buffer.cc
+++ b/base/trace_event/trace_buffer.cc
@@ -105,7 +105,7 @@ class TraceBufferRingBuffer : public TraceBuffer {
 
   void EstimateTraceMemoryOverhead(
       TraceEventMemoryOverhead* overhead) override {
-    overhead->Add("TraceBufferRingBuffer", sizeof(*this));
+    overhead->Add(TraceEventMemoryOverhead::kTraceBuffer, sizeof(*this));
     for (size_t queue_index = queue_head_; queue_index != queue_tail_;
          queue_index = NextQueueIndex(queue_index)) {
       size_t chunk_index = recyclable_chunks_queue_[queue_index];
@@ -221,7 +221,8 @@ class TraceBufferVector : public TraceBuffer {
         sizeof(*this) + max_chunks_ * sizeof(decltype(chunks_)::value_type);
     const size_t chunks_ptr_vector_resident_size =
         sizeof(*this) + chunks_.size() * sizeof(decltype(chunks_)::value_type);
-    overhead->Add("TraceBufferVector", chunks_ptr_vector_allocated_size,
+    overhead->Add(TraceEventMemoryOverhead::kTraceBuffer,
+                  chunks_ptr_vector_allocated_size,
                   chunks_ptr_vector_resident_size);
     for (size_t i = 0; i < chunks_.size(); ++i) {
       TraceBufferChunk* chunk = chunks_[i].get();
@@ -268,12 +269,13 @@ void TraceBufferChunk::EstimateTraceMemoryOverhead(
 
     // When estimating the size of TraceBufferChunk, exclude the array of trace
     // events, as they are computed individually below.
-    cached_overhead_estimate_->Add("TraceBufferChunk",
+    cached_overhead_estimate_->Add(TraceEventMemoryOverhead::kTraceBufferChunk,
                                    sizeof(*this) - sizeof(chunk_));
   }
 
   const size_t num_cached_estimated_events =
-      cached_overhead_estimate_->GetCount("TraceEvent");
+      cached_overhead_estimate_->GetCount(
+          TraceEventMemoryOverhead::kTraceEvent);
   DCHECK_LE(num_cached_estimated_events, size());
 
   if (IsFull() && num_cached_estimated_events == size()) {
@@ -291,7 +293,7 @@ void TraceBufferChunk::EstimateTraceMemoryOverhead(
     // changing as new TraceEvents are added to this chunk, so they are
     // computed on the fly.
     const size_t num_unused_trace_events = capacity() - size();
-    overhead->Add("TraceEvent (unused)",
+    overhead->Add(TraceEventMemoryOverhead::kUnusedTraceEvent,
                   num_unused_trace_events * sizeof(TraceEvent));
   }