Bump the branch used in the AOSP bot to include SDK v18.

scripts/slave/recipes/android_webview_aosp.expected/trybot.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS_ROOT]/gclient.py",
       "config",
       "--spec",
       "cache_dir = None\ndeps_os = {}\nhooks = []\nsolutions = [{'custom_deps': {}, 'custom_vars': {'googlecode_url': 'svn://svn-mirror.golo.chromium.org/%s', 'nacl_trunk': 'svn://svn-mirror.golo.chromium.org/native_client/trunk', 'sourceforge_url': 'svn://svn-mirror.golo.chromium.org/%(repo)s', 'webkit_trunk': 'svn://svn-mirror.golo.chromium.org/blink/trunk'}, 'deps_file': '', 'managed': True, 'name': 'src', 'safesync_url': None, 'url': 'svn://svn-mirror.golo.chromium.org/chrome/trunk/src'}]\ntarget_os = []\ntarget_os_only = False"
     ],
@@ skipping 148 matching lines @@
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "cmd": [
       "[DEPOT_TOOLS_ROOT]/repo",
       "init",
       "-u",
       "https://android.googlesource.com/platform/manifest",
       "-b",
-      "android-4.3_r2.1"
+      "android-4.3_r2.3"
     ],
     "cwd": "[SLAVE_BUILD_ROOT]/android-src",
     "name": "repo init"
   },
   {
     "cmd": [
       "[SLAVE_BUILD_ROOT]/src/android_webview/buildbot/generate_local_manifest.py",
       "[SLAVE_BUILD_ROOT]/android-src",
       "external/chromium_org"
     ],
@@ skipping 45 matching lines @@
       "--src-dir",
       "[SLAVE_BUILD_ROOT]/android-src",
       "--build-tool",
       "make-android",
       "--verbose"
     ],
     "cwd": "[SLAVE_BUILD_ROOT]",
     "name": "compile"
   }
 ]