[tracing] Dump context counts in v8 memory dump provider.

This patch adds the following dumps in v8 memory dump provider:
- v8/isolate/contexts/detached_context
- v8/isolate/contexts/native_context
Both dumps contain an "object_count" scalar value.

The value of detached_context is the number of contexts that were
detached (due to iframe navigation or removal) and not yet garbage
collected. This number being non-zero indicates potential memory leak.

The value of native_context is the number of the top-level contexts
created for the main window and iframes. Increase of this number
over time indicates a memory leak.

Bug: 793789
Change-Id: I4753454747c398fac4fc153fcccb1288d28f2a2d
Reviewed-on: https://chromium-review.googlesource.com/822336
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524438}

base/trace_event/memory_infra_background_whitelist.cc

@@ -179,6 +179,8 @@ const char* const kAllocatorDumpNameWhitelist[] = {
     "skia/sk_resource_cache",
     "sqlite",
     "ui/resource_manager_0x?",
+    "v8/isolate_0x?/contexts/detached_context",
+    "v8/isolate_0x?/contexts/native_context",
     "v8/isolate_0x?/heap_spaces",
     "v8/isolate_0x?/heap_spaces/code_space",
     "v8/isolate_0x?/heap_spaces/large_object_space",

gin/v8_isolate_memory_dump_provider.cc

@@ -81,6 +81,36 @@ void DumpCodeStatistics(
       code_statistics.bytecode_and_metadata_size());
 }
 
+// Dump the number of native and detached contexts.
+// The result looks as follows in the Chrome trace viewer:
+// ======================================
+// Component                 object_count
+// - v8
+//   - isolate
+//     - contexts
+//       - detached_context  10
+//       - native_context    20
+// ======================================
+void DumpContextStatistics(
+    base::trace_event::ProcessMemoryDump* process_memory_dump,
+    std::string dump_base_name,
+    size_t number_of_detached_contexts,
+    size_t number_of_native_contexts) {
+  std::string dump_name_prefix = dump_base_name + "/contexts";
+  std::string native_context_name = dump_name_prefix + "/native_context";
+  auto* native_context_dump =
+      process_memory_dump->CreateAllocatorDump(native_context_name);
+  native_context_dump->AddScalar(
+      "object_count", base::trace_event::MemoryAllocatorDump::kUnitsObjects,
+      number_of_native_contexts);
+  std::string detached_context_name = dump_name_prefix + "/detached_context";
+  auto* detached_context_dump =
+      process_memory_dump->CreateAllocatorDump(detached_context_name);
+  detached_context_dump->AddScalar(
+      "object_count", base::trace_event::MemoryAllocatorDump::kUnitsObjects,
+      number_of_detached_contexts);
+}
+
 }  // namespace anonymous
 
 void V8IsolateMemoryDumpProvider::DumpHeapStatistics(
@@ -176,6 +206,10 @@ void V8IsolateMemoryDumpProvider::DumpHeapStatistics(
                                           system_allocator_name);
   }
 
+  DumpContextStatistics(process_memory_dump, dump_base_name,
+                        heap_statistics.number_of_detached_contexts(),
+                        heap_statistics.number_of_native_contexts());
+
   // Add an empty row for the heap_spaces. This is to keep the shape of the
   // dump stable, whether code stats are enabled or not.
   auto* heap_spaces_dump =

gin/v8_isolate_memory_dump_provider_unittest.cc

@@ -52,4 +52,30 @@ TEST_F(V8MemoryDumpProviderTest, DumpStatistics) {
   ASSERT_TRUE(did_dump_objects_stats);
 }
 
+TEST_F(V8MemoryDumpProviderTest, DumpContextStatistics) {
+  base::trace_event::MemoryDumpArgs dump_args = {
+      base::trace_event::MemoryDumpLevelOfDetail::LIGHT};
+  std::unique_ptr<base::trace_event::ProcessMemoryDump> process_memory_dump(
+      new base::trace_event::ProcessMemoryDump(nullptr, dump_args));
+  instance_->isolate_memory_dump_provider_for_testing()->OnMemoryDump(
+      dump_args, process_memory_dump.get());
+  const base::trace_event::ProcessMemoryDump::AllocatorDumpsMap&
+      allocator_dumps = process_memory_dump->allocator_dumps();
+
+  bool did_dump_detached_contexts = false;
+  bool did_dump_native_contexts = false;
+  for (const auto& it : allocator_dumps) {
+    const std::string& dump_name = it.first;
+    if (dump_name.find("contexts/detached_context") != std::string::npos) {
+      did_dump_detached_contexts = true;
+    }
+    if (dump_name.find("contexts/native_context") != std::string::npos) {
+      did_dump_native_contexts = true;
+    }
+  }
+
+  ASSERT_TRUE(did_dump_detached_contexts);
+  ASSERT_TRUE(did_dump_native_contexts);
+}
+
 }  // namespace gin