[Android] Add output_file property to logcat monitor.
[Android] Add output_file property to logcat monitor.

Adding this for convienance to access logcat output if you
happen to want to do things with the files (like merge them)
after recording. This way to just don't have to store the file
names alongside the logcat_monitors.

BUG=

Review URL: https://codereview.chromium.org/1668703002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#373569}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk