Emit logcats from content_browsertests.

BUG=525873

Review URL: https://codereview.chromium.org/1637733002

Cr-Commit-Position: refs/heads/master@{#371457}

content/content_browsertests_apk.isolate

@@ -10,6 +10,7 @@
   'variables': {
     'command': [
       '<(PRODUCT_DIR)/bin/run_content_browsertests',
+      '--logcat-output-dir', '${ISOLATED_OUTDIR}/logcats',
     ],
     'files': [
       '../base/base.isolate',