Label /proc/meminfo.

Address the following denial:
m.chrome.canary: type=1400 audit(0.0:15): avc: granted { read open } for path="/proc/meminfo" dev="proc" ino=4026544360 scontext=u:r:untrusted_app:s0:c512,c768 tcontext=u:object_r:proc:s0 tclass=file

(cherry-pick of internal commit: 971aeeda)

Bug: 22032619
Chromium Bug: 586021

Change-Id: I2dcb2d4800bbc92ea47c37d4fd7a10f827a0114c