Replace DBus method constant with system_api constant

"GetExistingDlcs" DBus method constant is in system_api now and can be
used within dlcservice client files.

BUG=none
TEST=./unit_tests

Change-Id: I347fbabef4691f69ec3482ffbf51cbe7c9b54c59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2199679
Auto-Submit: Jae Hoon Kim <kimjae@chromium.org>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#768511}

chromeos/dbus/dlcservice/dlcservice_client.cc

@@ -37,9 +37,6 @@ namespace chromeos {
 
 namespace {
 
-// TODO(kimjae): Delete and remove usages once system_api is uprev'ed.
-const char kGetExistingDlcsMethod[] = "GetExistingDlcs";
-
 DlcserviceClient* g_instance = nullptr;
 
 class DlcserviceErrorResponseHandler {
@@ -192,9 +189,8 @@ class DlcserviceClientImpl : public DlcserviceClient {
     }
 
     TaskStarted();
-    // TODO(kimjae): Use |kGetExistingDlcsMethod| once system_api is uprev'ed.
     dbus::MethodCall method_call(dlcservice::kDlcServiceInterface,
-                                 kGetExistingDlcsMethod);
+                                 dlcservice::kGetExistingDlcsMethod);
 
     VLOG(1) << "Requesting to get existing DLC(s).";
     dlcservice_proxy_->CallMethodWithErrorResponse(

chromeos/dbus/dlcservice/dlcservice_client_unittest.cc

@@ -125,9 +125,8 @@ TEST_F(DlcserviceClientTest, GetExistingDlcsSuccessTest) {
 }
 
 TEST_F(DlcserviceClientTest, GetExistingDlcsFailureTest) {
-  // TODO(kimjae): Use |kGetExistingDlcsMethod| once system_api is uprev'ed.
   dbus::MethodCall method_call(dlcservice::kDlcServiceInterface,
-                               "GetExsitingDlcs");
+                               dlcservice::kGetExistingDlcsMethod);
   method_call.SetSerial(123);
   err_responses_.push_back(dbus::ErrorResponse::FromMethodCall(
       &method_call, DBUS_ERROR_FAILED, "some-unknown-error"));