Enable multidex for debug builds of system_webview_apk_tmpl-based apks.

android_webview/system_webview_apk_tmpl.gni

 # Copyright 2015 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/android/rules.gni")
 import("//build/config/locales.gni")
 
 template("system_webview_apk_tmpl") {
   android_apk(target_name) {
@@ skipping 12 matching lines @@
     shared_libraries = [ "//android_webview:libwebviewchromium" ]
     native_lib_version_rule = "//build/util:chrome_version_json"
     _native_lib_file =
         rebase_path("$root_gen_dir/CHROME_VERSION.json", root_out_dir)
     native_lib_version_arg = "@FileArg($_native_lib_file:full-quoted)"
     if (build_apk_secondary_abi && android_64bit_target_cpu) {
       secondary_abi_shared_libraries = [ "//android_webview:libwebviewchromium($android_secondary_abi_toolchain)" ]
     }
     aapt_locale_whitelist = locales
 
-    if (!is_java_debug) {
+    if (is_java_debug) {
+      enable_multidex = true
+    } else {
       proguard_enabled = true
       if (!defined(proguard_configs)) {
         proguard_configs = []
       }
       proguard_configs += [
         "//android_webview/apk/java/proguard.flags",
         "//base/android/proguard/chromium_apk.flags",
         "//base/android/proguard/chromium_code.flags",
       ]
     }
   }
 }