update_engine: Place enterprise rollback save marker file

- There are now two types of rollback. One that does a full powerwash
  and one that save some system state.
- When an enterprise rollback powerwash is scheduled, place a marker
  file to tell the shutdown process to save data before rebooting.
- This lets rollback preserve additional data over a powerwash that
  can be restored later.
- Change a few places that were using is_rollback to save_rollback_data
  to be explicit.

BUG=chromium:955463
TEST=unittests

Change-Id: I9f18319e711e425a6e712dd319e03bcc6ddd0a1b
Reviewed-on: https://chromium-review.googlesource.com/1414030
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/common/fake_hardware.h b/common/fake_hardware.h
index 3e5a66e..53b2dd5 100644
--- a/common/fake_hardware.h
+++ b/common/fake_hardware.h
@@ -104,15 +104,15 @@
 
   int GetPowerwashCount() const override { return powerwash_count_; }
 
-  bool SchedulePowerwash(bool is_rollback) override {
+  bool SchedulePowerwash(bool save_rollback_data) override {
     powerwash_scheduled_ = true;
-    is_rollback_powerwash_ = is_rollback;
+    save_rollback_data_ = save_rollback_data;
     return true;
   }
 
   bool CancelPowerwash() override {
     powerwash_scheduled_ = false;
-    is_rollback_powerwash_ = false;
+    save_rollback_data_ = false;
     return true;
   }
 
@@ -193,7 +193,7 @@
   int GetMaxKernelKeyRollforward() const { return kernel_max_rollforward_; }
 
   bool GetIsRollbackPowerwashScheduled() const {
-    return powerwash_scheduled_ && is_rollback_powerwash_;
+    return powerwash_scheduled_ && save_rollback_data_;
   }
 
  private:
@@ -213,7 +213,7 @@
   int firmware_max_rollforward_{kFirmwareMaxRollforward};
   int powerwash_count_{kPowerwashCountNotSet};
   bool powerwash_scheduled_{false};
-  bool is_rollback_powerwash_{false};
+  bool save_rollback_data_{false};
   int64_t build_timestamp_{0};
   bool first_active_omaha_ping_sent_{false};