Add authorize_adb_keys step that will add host key to device.

scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_android.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "tryserver.webrtc",
       "--builder",
       "android",
@@ skipping 114 matching lines @@
       "[SLAVE_BUILD]/src/build/android/adb_logcat_monitor.py",
       "[SLAVE_BUILD]/src/out/logcat"
     ],
     "env": {
       "PATH": "[SLAVE_BUILD]/src/third_party/android_tools/sdk/platform-tools:[SLAVE_BUILD]/src/build/android:%(PATH)s"
     },
     "name": "spawn_logcat_monitor"
   },
   {
     "cmd": [
+      "python",
+      "-u",
+      "[BUILD]/scripts/slave/android/authorize_adb_devices.py",
+      "--adb-path",
+      "[SLAVE_BUILD]/src/third_party/android_tools/sdk/platform-tools/adb"
+    ],
+    "env": {
+      "PATH": "[SLAVE_BUILD]/src/third_party/android_tools/sdk/platform-tools:[SLAVE_BUILD]/src/build/android:%(PATH)s"
+    },
+    "name": "authorize_adb_devices"
+  },
+  {
+    "cmd": [
       "[SLAVE_BUILD]/src/build/android/buildbot/bb_device_status_check.py",
       "--json-output",
       "/path/to/tmp/json"
     ],
     "env": {
       "PATH": "[SLAVE_BUILD]/src/third_party/android_tools/sdk/platform-tools:[SLAVE_BUILD]/src/build/android:%(PATH)s"
     },
     "name": "device_status_check",
     "~followup_annotations": [
       "@@@STEP_TEXT@Online devices: 1@@@",
@@ skipping 268 matching lines @@
       "@@@STEP_LOG_LINE@python.inline@import glob, os, sys@@@",
       "@@@STEP_LOG_LINE@python.inline@for report in glob.glob(sys.argv[1]):@@@",
       "@@@STEP_LOG_LINE@python.inline@  with open(report, 'r') as f:@@@",
       "@@@STEP_LOG_LINE@python.inline@    for l in f.readlines():@@@",
       "@@@STEP_LOG_LINE@python.inline@      print l@@@",
       "@@@STEP_LOG_LINE@python.inline@  os.remove(report)@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   }
 ]