Fix gn gen on //printing when use_sysroot = false

printing/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/features.gni")
 import("//build/config/sysroot.gni")
 import("//build/config/ui.gni")
 import("//testing/test.gni")
 if (is_mac) {
   import("//build/config/mac/mac_sdk.gni")
@@ skipping 110 matching lines @@
     sources += [
       "printing_context_no_system_dialog.cc",
       "printing_context_no_system_dialog.h",
     ]
   }
 
   if (use_cups) {
     configs += [ ":cups" ]
 
     if (is_linux) {
+      # rebase_path does not accept an empty string
+      if (use_sysroot) {
+        cups_sysroot = rebase_path(sysroot)
+      } else {
+        cups_sysroot = ""
+      }
       cups_version = exec_script("cups_config_helper.py",
                                  [
                                    "--api-version",
-                                   rebase_path(sysroot),
+                                   cups_sysroot,
                                  ],
                                  "trim string")
 
       if (cups_version == "1.6" || cups_version == "1.7") {
         cflags += [
           # CUPS 1.6 deprecated the PPD APIs, but we will stay with this
           # API for now as supported Linux and Mac OS'es are still using
           # older versions of CUPS. More info: crbug.com/226176
           "-Wno-deprecated-declarations",
           # CUPS 1.7 deprecates httpConnectEncrypt(), see the mac section
@@ skipping 89 matching lines @@
 }
 
 if (use_cups) {
   config("cups") {
     defines = [ "USE_CUPS" ]
 
     if (is_mac) {
       libs = [ "cups" ]
       lib_dirs = [ "$mac_sdk_path/usr/lib" ]
     } else {
+      # rebase_path does not accept an empty string
+      if (use_sysroot) {
+        cups_sysroot = rebase_path(sysroot)
+      } else {
+        cups_sysroot = ""
+      }
       libs = exec_script("cups_config_helper.py",
                          [
                            "--libs-for-gn",
-                           rebase_path(sysroot),
+                           cups_sysroot,
                          ],
                          "value")
     }
   }
 }
 
 if (is_android) {
   # GYP: //printing/printing.gyp:printing_jni_headers
   generate_jni("printing_jni_headers") {
     sources = [
       "android/java/src/org/chromium/printing/PrintingContext.java",
     ]
     jni_package = "printing"
   }
 
   # GYP: //printing/printing.gyp:printing_java
   android_library("printing_java") {
     deps = [
       "//base:base_java",
     ]
     DEPRECATED_java_in_dir = "android/java/src"
   }
 }