Regularize ownerships and lifecycle for storage monitor platform classes.

chrome/browser/storage_monitor/storage_monitor.h

Index: chrome/browser/storage_monitor/storage_monitor.h
diff --git a/chrome/browser/storage_monitor/storage_monitor.h b/chrome/browser/storage_monitor/storage_monitor.h
index 86ef72c17441fd4b541d3f2324373f60f3924d63..17e578285de1453afb2f311c594c944e195f4c59 100644
--- a/chrome/browser/storage_monitor/storage_monitor.h
+++ b/chrome/browser/storage_monitor/storage_monitor.h
@@ -24,6 +24,11 @@ class TransientDeviceIds;
 
 // Base class for platform-specific instances watching for removable storage
 // attachments/detachments.
+// Lifecycle contracts: This class is created by ChromeBrowserMain
+// implementations before the profile is initialized, so listeners can be
+// created during profile construction. The platform-specific initialization,
+// which can lead to calling registered listeners with notifications of
+// attached volumes, will happen after profile construction.
 class StorageMonitor {
  public:
   struct StorageInfo {