Use --incremental when dex'ing (when possible).

build/config/android/internal_rules.gni

 # 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/android/config.gni")
 import("//build/config/zip.gni")
 import("//third_party/ijar/ijar.gni")
 
 assert(is_android)
 
@@ skipping 181 matching lines @@
 
     args = [
       "--depfile",
       rebase_path(depfile, root_build_dir),
       "--android-sdk-tools",
       rebased_android_sdk_build_tools,
       "--dex-path",
       rebased_output,
     ]
 
+    if (incremental_dx) {
+      args += [ "--incremental" ]
+    }
+
     if (defined(invoker.no_locals) && invoker.no_locals) {
       args += [ "--no-locals=1" ]
     }
 
     if (defined(invoker.args)) {
       args += invoker.args
     }
 
     if (defined(invoker.sources)) {
       args += rebase_path(invoker.sources, root_build_dir)
@@ skipping 1439 matching lines @@
     ]
     args = [
       "--depfile",
       rebase_path(depfile, root_build_dir),
       "--script-output-path",
       rebase_path(generated_script, root_build_dir),
     ]
     args += test_runner_args
   }
 }