update_engine: add variable for DeviceQuickFixBuildToken policy

This CL makes DeviceQuickFixBuildToken policy visible in update_manager
which would be used in a follow up CL to skip applying
DeviceAutoUpdateTimeRestrictions when a quick fix build token is set.
This CL also adds quick fix token in UpdateCheckParams which is used
by UpdateAttempter to pass the token in OmahaRequestParams.

BUG=chromium:1117450
TEST=FEATURES=test emerge-${BOARD} update_engine

Change-Id: If43ad8cd2955bbeb1cbd1dcacac79742fa0a6a20
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2521189
Commit-Queue: Saurabh Nijhara <snijhara@google.com>
Tested-by: Saurabh Nijhara <snijhara@google.com>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
diff --git a/update_manager/device_policy_provider.h b/update_manager/device_policy_provider.h
index a59f2a3..5112f68 100644
--- a/update_manager/device_policy_provider.h
+++ b/update_manager/device_policy_provider.h
@@ -97,6 +97,10 @@
   // Chrome OS version number.
   virtual Variable<base::Version>* var_device_minimum_version() = 0;
 
+  // Variable that contains a token which maps to a Chrome OS Quick Fix Build to
+  // which the device would be updated if not blocked by another policy.
+  virtual Variable<std::string>* var_quick_fix_build_token() = 0;
+
  protected:
   DevicePolicyProvider() {}