Printing API: Add comments for MockPrintJobHistoryService class

Bug: 992889
Change-Id: I4d4c2f8dcd906e532754e2d306a5be3441324e48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1810997Reviewed-by: Sean Kau <skau@chromium.org>
Commit-Queue: Nikita Podguzov <nikitapodguzov@google.com>
Cr-Commit-Position: refs/heads/master@{#698756}

chrome/browser/chromeos/printing/history/mock_print_job_history_service.h

@@ -11,11 +11,14 @@
 
 namespace chromeos {
 
+// The mock implementation of PrintJobHistoryService for testing.
 class MockPrintJobHistoryService : public PrintJobHistoryService {
  public:
   MockPrintJobHistoryService();
   ~MockPrintJobHistoryService() override;
 
+  // This method doesn't save print job to the persistent storage.
+  // It should be used only for testing to notify observers.
   void SavePrintJobProto(const printing::proto::PrintJobInfo& print_job_info);
 
   MOCK_METHOD(void,