[Android] Clear adb logcat during adb_logcat_monitor startup.

BUG=249039
NOTRY=True

Review URL: https://chromiumcodereview.appspot.com/17438004

git-svn-id: http://src.chromium.org/svn/trunk/src/build@207175 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

android/adb_logcat_monitor.py

@@ -126,6 +126,7 @@ def main(base_dir, adb_cmd='adb'):
     while True:
       for device_id in GetAttachedDevices(adb_cmd):
         if not device_id in devices:
+          subprocess.call([adb_cmd, '-s', device_id, 'logcat', '-c'])
           devices[device_id] = (None, 0)
 
       for device in devices: