[Android] Add gn support for multidex.

chrome/android/chrome_public_apk_tmpl.gni

Index: chrome/android/chrome_public_apk_tmpl.gni
diff --git a/chrome/android/chrome_public_apk_tmpl.gni b/chrome/android/chrome_public_apk_tmpl.gni
index 5a6296c8af68545605493c5f2357b5e71c62edc8..26ae9fc735d711508cc1d22152401b9d4bfd0c99 100644
--- a/chrome/android/chrome_public_apk_tmpl.gni
+++ b/chrome/android/chrome_public_apk_tmpl.gni
@@ -23,7 +23,9 @@ template("chrome_public_apk_tmpl") {
         rebase_path("$root_gen_dir/CHROME_VERSION.json", root_out_dir)
     native_lib_version_arg = "@FileArg($_native_lib_file:full-quoted)"
 
-    if (!is_debug) {
+    if (is_debug) {
+      enable_multidex = true
+    } else {
       proguard_enabled = true
       _prev_proguard_configs = []
       if (defined(proguard_configs)) {