Gather metrics on hosted app unlimitedStorage usage

BUG=416691

Review URL: https://codereview.chromium.org/595683002

Cr-Commit-Position: refs/heads/master@{#296298}

extensions/browser/api/storage/local_value_store_cache.cc

@@ -9,6 +9,7 @@
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/files/file_path.h"
+#include "base/metrics/histogram.h"
 #include "content/public/browser/browser_thread.h"
 #include "extensions/browser/api/storage/settings_storage_factory.h"
 #include "extensions/browser/api/storage/settings_storage_quota_enforcer.h"
@@ -91,6 +92,16 @@ ValueStore* LocalValueStoreCache::GetStorage(
       new SettingsStorageQuotaEnforcer(
           quota_, storage_factory_->Create(file_path, extension->id())));
   DCHECK(storage.get());
+
+  if (extension->permissions_data()->HasAPIPermission(
+          APIPermission::kUnlimitedStorage) &&
+      extension->is_hosted_app()) {
+    // We're interested in the amount of space hosted apps are using. Record it
+    // the first time we load the storage for the extension.
+    UMA_HISTOGRAM_MEMORY_KB("Extensions.HostedAppUnlimitedStorageUsage",
+                            storage->GetBytesInUse());
+  }
+
   storage_map_[extension->id()] = storage;
   return storage.get();
 }

tools/metrics/histograms/histograms.xml

@@ -7677,6 +7677,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Extensions.HostedAppUnlimitedStorageUsage" units="KB">
+  <owner>rdevlin.cronin@chromium.org</owner>
+  <summary>
+    The usage (in kilobytes) of a hosted app with the unlimitedStorage api
+    permission, recorded once per run per app the first time we load storage for
+    the hosted app.
+  </summary>
+</histogram>
+
 <histogram name="Extensions.IncognitoAllowed">
   <owner>kalman@chromium.org</owner>
   <summary>