[tracing] Switch to new heap dump format.

base/trace_event/heap_profiler_string_deduplicator.cc

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/trace_event/heap_profiler_string_deduplicator.h"
+
+#include "base/trace_event/memory_usage_estimator.h"
+#include "base/trace_event/trace_event.h"
+#include "base/trace_event/trace_event_argument.h"
+#include "base/trace_event/trace_event_memory_overhead.h"
+
+namespace base {
+namespace trace_event {
+
+StringDeduplicator::StringDeduplicator() : last_exported_index_(0) {
+  // Add implicit entry for id 0 (NULL strings).
+  strings_.push_back("[null]");
+}
+
+StringDeduplicator::~StringDeduplicator() {}
+
+int StringDeduplicator::Insert(StringPiece string) {
+  if (!string.data()) {

[Primiano Tucci (use gerrit), 2017/03/09 11:47:45]

should this also be + || string.empty() to reuse the key also for empty strings?
Or we just expect that to not happen frequently enough?

[DmitrySkiba, 2017/03/14 22:12:47]

NULL is the only value that means "no type name", so it's handled specially.
Empty string doesn't make much sense as a type name, but still, it's something,
while NULL is nothing. 

+    // NULL strings are mapped to id 0.
+    return 0;
+  }
+  int next_id = static_cast<int>(strings_.size());
+  auto iter_and_flag = string_ids_.insert({string, next_id});
+  if (iter_and_flag.second) {
+    // Inserted a new mapping
+    strings_.push_back(string.as_string());

[Primiano Tucci (use gerrit), 2017/03/09 11:47:45]

I think that if you do
strings_.emplace_back(string.data()) here you might avoid an unnecessary string
copy here.
Alternatively, push_back(std::move(string.as_string)) but I never remember if
strings are move types and, even if they are, I wouldn't put my hand on fire
they are implemented sanely in all STL implementations :) 

[DmitrySkiba, 2017/03/14 22:12:47]

push_back() has && variant, which is used in this case, so push_back() behaves
the same as emplace_back() would.

+  }
+  return iter_and_flag.first->second;
+}
+
+void StringDeduplicator::ExportIncrementally(TracedValue* traced_value) {
+  for (; last_exported_index_ != strings_.size(); ++last_exported_index_) {
+    traced_value->BeginDictionary();
+    traced_value->SetInteger("id", last_exported_index_);
+    traced_value->SetString("string", strings_[last_exported_index_]);
+    traced_value->EndDictionary();
+  }
+}
+
+void StringDeduplicator::EstimateTraceMemoryOverhead(
+    TraceEventMemoryOverhead* overhead) {
+  size_t memory_usage =
+      EstimateMemoryUsage(string_ids_) + EstimateMemoryUsage(strings_);
+  overhead->Add("StringDeduplicator",
+                sizeof(StringDeduplicator) + memory_usage);
+}
+
+}  // namespace trace_event
+}  // namespace base