Sentence case for [My|Linux|Play] files

Essentially changing:
 * My Files => My files.
 * Linux Files => Linux files.
 * Play Files => Play files.


Bug: 864299
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: Ic3c614f654ae0a7cfaa5efb67d6f966acfccc721
Reviewed-on: https://chromium-review.googlesource.com/1139456
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575558}
7 files changed