[Android] Rework multidex and enable multidex for unit_tests_apk.

chrome/chrome_tests_unit.gypi

 # Copyright 2012 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.
 {
   'variables': {
     'chrome_unit_tests_sources': [
       # histograms.xml is analyzed by AboutFlagsHistogramTest, so this
       # dependency is needed to make commit bots run unit_tests on
       # histograms.xml changes.
       '../tools/metrics/histograms/histograms.xml',
@@ skipping 2966 matching lines @@
           'type': 'none',
           'dependencies': [
             'chrome_java',
             'unit_tests_java',
             'unit_tests',
           ],
           'variables': {
             'test_suite_name': 'unit_tests',
             'isolate_file': 'unit_tests.isolate',
             'android_manifest_path': 'test/android/unit_tests_apk/AndroidManifest.xml',
+            'enable_multidex': 1,
             'conditions': [
               ['v8_use_external_startup_data==1', {
                 'asset_location': '<(PRODUCT_DIR)/unit_tests_apk/assets',
                 'additional_input_paths': [
                   '<(PRODUCT_DIR)/unit_tests_apk/assets/natives_blob_<(arch_suffix).bin',
                   '<(PRODUCT_DIR)/unit_tests_apk/assets/snapshot_blob_<(arch_suffix).bin',
                 ],
               }],
             ],
           },
@@ skipping 62 matching lines @@
               'dependencies': [
                 '../tools/xdisplaycheck/xdisplaycheck.gyp:xdisplaycheck',
               ],
             }],
           ],
         },
       ],
     }],
   ],  # 'conditions'
 }