Fix memlog JSON export when Perfetto is enabled

Added a switch to force JSON-backed TracedValue writer mode and
switch memlog to use it.

R=erikchen@chromium.org,ssid@chromium.org

Bug: 839084
Change-Id: I456b4a225112205993299ad98b39c75b55d59ca3
Reviewed-on: https://chromium-review.googlesource.com/c/1480825Reviewed-by: ssid <ssid@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634323}

base/trace_event/traced_value.cc

@@ -456,10 +456,11 @@ void TracedValue::SetWriterFactoryCallback(WriterFactoryCallback callback) {
 
 TracedValue::TracedValue() : TracedValue(0) {}
 
-TracedValue::TracedValue(size_t capacity) {
+TracedValue::TracedValue(size_t capacity, bool force_json) {
   DEBUG_PUSH_CONTAINER(kStackTypeDict);
 
-  writer_ = CreateWriter(capacity);
+  writer_ = force_json ? std::make_unique<PickleWriter>(capacity)
+                       : CreateWriter(capacity);
 }
 
 TracedValue::~TracedValue() {

base/trace_event/traced_value.h

@@ -25,7 +25,7 @@ namespace trace_event {
 class BASE_EXPORT TracedValue : public ConvertableToTraceFormat {
  public:
   TracedValue();
-  explicit TracedValue(size_t capacity);
+  explicit TracedValue(size_t capacity, bool force_json = false);
   ~TracedValue() override;
 
   void EndDictionary();

components/services/heap_profiling/json_exporter.cc

@@ -160,7 +160,7 @@ void WriteHeapsV2Footer(std::ostream& out) {
 }
 
 void WriteMemoryMaps(const ExportParams& params, std::ostream& out) {
-  base::trace_event::TracedValue traced_value;
+  base::trace_event::TracedValue traced_value(0, /*force_json=*/true);
   memory_instrumentation::TracingObserver::MemoryMapsAsValueInto(
       params.maps, &traced_value, params.strip_path_from_mapped_files);
   out << "\"process_mmaps\":" << traced_value.ToString();