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

build/config/android/internal_rules.gni

Index: build/config/android/internal_rules.gni
diff --git a/build/config/android/internal_rules.gni b/build/config/android/internal_rules.gni
index e1b3832a187531057865e56300fbcb594d7b0c41..05c6bcbbec34c73c91ece1b633d23ccb3df6ba41 100644
--- a/build/config/android/internal_rules.gni
+++ b/build/config/android/internal_rules.gni
@@ -199,6 +199,10 @@ template("dex") {
       rebased_output,
     ]
 
+    if (incremental_dx) {
+      args += [ "--incremental" ]
+    }
+
     if (defined(invoker.no_locals) && invoker.no_locals) {
       args += [ "--no-locals=1" ]
     }