Add /proc/stat and /proc/meminfo to tts service read-only files

Required to support
https://chromium-review.googlesource.com/c/chromium/src/+/2510736

R=mpdenton@chromium.org

Test: on patched .so, and patched build of Chrome without any further modification, and a standard .so build using chromiumos toolchain, verify tts service works on arm32.
Change-Id: Idfefa22d2819ce819f3a265f4fa3e7169e6697ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518729


Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Auto-Submit: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824927}