Fix MB configs for chromium.webkit Android GN bots.

... which weren't specifying android builds :).

R=phajdan.jr@chromium.org
BUG=454413

Review URL: https://codereview.chromium.org/1090053002

Cr-Commit-Position: refs/heads/master@{#325559}

tools/mb/mb_config.pyl

@@ -154,8 +154,8 @@
       'Win8 GN (dbg)': 'gn_debug_static_bot_x86',
     },
     'chromium.webkit': {
-      'Android GN': 'gn_release_bot',
-      'Android GN (dbg)': 'gn_debug_bot',
+      'Android GN': 'android_gn_release_bot',
+      'Android GN (dbg)': 'android_gn_debug_bot',
       'Linux GN': 'gn_release_bot',
       'Linux GN (dbg)': 'gn_debug_bot'
     },