Emit logcats from chrome public tests.

BUG=525873

Review URL: https://codereview.chromium.org/1638583005

Cr-Commit-Position: refs/heads/master@{#371643}

chrome/android/chrome_public_test_apk.isolate

@@ -12,6 +12,7 @@
       '--enable-platform-mode',
       '-e', 'local', '--apk-under-test',
       '<(PRODUCT_DIR)/apks/ChromePublic.apk',
+      '--logcat-output-dir', '${ISOLATED_OUTDIR}/logcats',
     ],
     'files': [
       '../../third_party/proguard/',