[Android] Rework multidex and enable multidex for unit_tests_apk.

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index 5d8510f1b49dae4087fa782edf1d687cb6333611..1265df839b53b17168150bb5feda42b25f619553 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -2057,9 +2057,10 @@ if (is_android) {
 
     DEPRECATED_java_in_dir = "android/java/src"
 
-    # A new version of NativeLibraries.java (with the actual correct values)
-    # will be created when creating an apk.
+    # New versions of ChromiumMultiDex.java and NativeLibraries.java
+    # (with the actual correct values) will be created when creating an apk.
     jar_excluded_patterns = [
+      "*/ChromiumMultiDex.class",
       "*/NativeLibraries.class",
       "*/NativeLibraries##*.class",
     ]
@@ -2094,6 +2095,7 @@ if (is_android) {
       "//third_party/robolectric:android-all-4.3_r2-robolectric-0",
       "//third_party/robolectric:robolectric_java",
     ]
+    srcjar_deps = [ ":base_multidex_gen" ]
   }
 
   # GYP: //base.gyp:base_junit_tests
@@ -2128,9 +2130,6 @@ if (is_android) {
     sources = [
       "android/java/templates/ChromiumMultiDex.template",
     ]
-    if (is_debug) {
-      defines = [ "MULTIDEX_CONFIGURATION_Debug" ]
-    }
     package_name = "org/chromium/base/multidex"
   }