iOS: Specify device to run perf tests.

It expects 'bot id', not 'bot_id'. As it is now, this is just ignored.

TBR=phoglund@webrtc.org

No-Try: true
Bug: webrtc:7156
Change-Id: I58443df82655bef5a88a416e4fa5e080ddb93772
Reviewed-on: https://webrtc-review.googlesource.com/38420
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Edward Lemur <ehmaldonado@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21579}

tools_webrtc/ios/tests/perf_tests.json

@@ -5,7 +5,7 @@
       "test args": [
         "--save_chartjson_result"
       ],
-      "bot_id": "build15-a7",
+      "bot id": "build15-a7",
       "pool": "WebRTC"
     }
   ]

tools_webrtc/ios/tests/perf_trybot_tests.json

@@ -5,7 +5,7 @@
       "test args": [
         "--save_chartjson_result"
       ],
-      "bot_id": "build16-a7",
+      "bot id": "build16-a7",
       "pool": "WebRTC"
     }
   ]