Debug built crashes on start using Linux kernel > 3.16 due to SHMMAX change

https://code.google.com/p/chromium/issues/detail?id=403869
Cherry-pick https://codereview.chromium.org/471893003
Author: brettw@chromium.org

Parse shmmax as a uint64

A recent Linux kernel change changed this to a number larger than 63 bits which causes us to compute 0 size.
I kept the size_t overflow check for cases where size_t is 32-bits.

BUG=403869
R=jamesr@chromium.org

Change-Id: I2ed233ec7d98484e3b25534df3a087d4f8ea26b4
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=289903
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>