Create and integrate a metrics service client into Blimp engine.

blimp/engine/common/blimp_browser_context.cc

Index: blimp/engine/common/blimp_browser_context.cc
diff --git a/blimp/engine/common/blimp_browser_context.cc b/blimp/engine/common/blimp_browser_context.cc
index 6a2a7ec04ad9ba83d257b766bf9ef850515804b0..5ab92b654259cbbfc02b536fdf28271962d045e1 100644
--- a/blimp/engine/common/blimp_browser_context.cc
+++ b/blimp/engine/common/blimp_browser_context.cc
@@ -7,14 +7,27 @@
 #include "base/bind.h"
 #include "base/environment.h"
 #include "base/files/file_util.h"
+#include "base/memory/ptr_util.h"
 #include "base/nix/xdg_util.h"
 #include "base/path_service.h"
 #include "blimp/engine/app/blimp_permission_manager.h"
+#include "components/metrics/metrics_service.h"
+#include "components/pref_registry/pref_registry_syncable.h"
+#include "components/prefs/in_memory_pref_store.h"
+#include "components/prefs/pref_service.h"
+#include "components/prefs/pref_service_factory.h"
 #include "content/public/browser/background_sync_controller.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/storage_partition.h"
 #include "content/public/common/content_switches.h"
 
+namespace {
+// Function for optionally handling read errors. Is a no-op for Blimp.
+// While the PersistentPrefStore's interface is supported, it is an in-memory
+// store only.
+void HandleReadError(PersistentPrefStore::PrefReadError error) {}

[Wez, 2016/05/01 00:12:59]

Feels like this comment could be clearer e.g. "Callback used to handle PrefStore
read errors; since we use an in-memory store we don't expect any errors." and
then have the implementation body be NOTREACHED().

+}  // namespace
+
 namespace blimp {
 namespace engine {
 
@@ -68,6 +81,14 @@ BlimpBrowserContext::~BlimpBrowserContext() {
   }
 }
 
+void BlimpBrowserContext::Initialize() {
+  InitializeBlimpMetrics(GetPrefService(), GetSystemRequestContextGetter());

[Wez, 2016/05/01 00:12:59]

Is there a reason that these Initialize and Finalize can't be folded into the
constructor & destructor? e.g. do they need to be called at very specific point
during startup & shutdown? If so then consider naming them more specifically
e.g. OnBrowserMainPartsInitialize() or DoBrowserMainPartsInit() or even just
InitializeMetrics().

+}
+
+void BlimpBrowserContext::Finalize() {
+  FinalizeBlimpMetrics();
+}
+
 void BlimpBrowserContext::InitWhileIOAllowed() {
   // Ensures ~/.config/blimp_engine directory exists.
   std::unique_ptr<base::Environment> env(base::Environment::Create());
@@ -79,6 +100,20 @@ void BlimpBrowserContext::InitWhileIOAllowed() {
   BrowserContext::Initialize(this, path_);
 }
 
+std::unique_ptr<PrefService> BlimpBrowserContext::GetPrefService() {

[Wez, 2016/05/01 00:12:59]

nit: Put blank lines between each code-block and the comment starting the next
code block, for readability.

+  // Create PrefRegistry and register metrics services preferences with it.
+  scoped_refptr<user_prefs::PrefRegistrySyncable> pref_registry(
+      new user_prefs::PrefRegistrySyncable());
+  metrics::MetricsService::RegisterPrefs(pref_registry.get());
+  PrefServiceFactory pref_service_factory;
+  // Create an in memory preferences store to hold metrics logs.

[Wez, 2016/05/01 00:12:59]

i.e. move this comment to precede declaration of |pref_service_factory| and put
a blank line between it and the preceding RegisterPrefs() call.

+  pref_service_factory.set_user_prefs(new InMemoryPrefStore());
+  pref_service_factory.set_read_error_callback(base::Bind(&HandleReadError));
+  // Create a PrefService binding the PrefRegistry to the InMemoryPrefStore.
+  // The PrefService ends up owning the PrefRegistry and the InMemoryPrefStore.

[Wez, 2016/05/01 00:12:59]

What do we mean by "ends up owning"? Since PrefRegistry is ref-counted it looks
like the PrefService holds a _reference_ to it. Similarly the InMemoryPrefStore
is also ref-counted, so the PrefService presumably only takes a reference to it,
rather than owning it (which makes sense since the InMemoryPrefStore is passed
to the PrefServiceFactory and so presumably might be associated with several
PrefService instances).

+  return pref_service_factory.Create(pref_registry.get());
+}
+
 std::unique_ptr<content::ZoomLevelDelegate>
 BlimpBrowserContext::CreateZoomLevelDelegate(const base::FilePath&) {
   return nullptr;