Emit logcats from device_unittests.

BUG=525873

Review URL: https://codereview.chromium.org/1634023003

Cr-Commit-Position: refs/heads/master@{#371458}

device/device_unittests_apk.isolate

@@ -8,6 +8,7 @@
   'variables': {
     'command': [
       '<(PRODUCT_DIR)/bin/run_device_unittests',
+      '--logcat-output-dir', '${ISOLATED_OUTDIR}/logcats',
     ],
     'files': [
       '<(PRODUCT_DIR)/bin/run_device_unittests',