WebRTC: Save logcat output to isolated outputs.

NOTRY=True
BUG=chromium:717417

Review-Url: https://codereview.webrtc.org/3008343002
Cr-Commit-Position: refs/heads/master@{#19748}

tools_webrtc/mb/mb.py

@@ -1057,7 +1057,10 @@ class MetaBuildWrapper(object):
     if android:
       cmdline = ['../../build/android/test_wrapper/logdog_wrapper.py',
                  '--target', target,
-                 '--logdog-bin-cmd', '../../bin/logdog_butler']
+                 '--logdog-bin-cmd', '../../bin/logdog_butler',
+                 '--logcat-output-file', '${ISOLATED_OUTDIR}/logcats',
+                 '--store-tombstones']
+
       if test_type != 'junit_test':
         cmdline += ['--target-devices-file', '${SWARMING_BOT_FILE}']
     else: