Update compiler warning flags

Update compiler warning flags

 - add -Wsign-compare, which has been catching useful issues for Kimmo;
 - add -Winit-self and -Wpointer-arith to Mac builds so everyone's using
   the same flags;
 - try try removing -Wno-uninitialized.  This was only for the old 10.6
   compiler that we have warnings set as non-errors now.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/a07b297585f58c4cb4144e5625a31ce5d98c5e04

[mtklein, 2015-01-23 14:47:18]

The CQ bit was checked by mtklein@google.com

[mtklein, 2015-01-23 14:47:34]

mtklein@google.com changed reviewers:
+ bsalomon@google.com

[commit-bot: I haz the power, 2015-01-23 14:48:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/872793002/1

[commit-bot: I haz the power, 2015-01-23 14:48:21]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2015-01-23 20:48 UTC

[bsalomon, 2015-01-23 14:58:53]

lgtm

[commit-bot: I haz the power, 2015-01-23 15:01:29]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/a07b297585f58c4cb4144e5625a31ce5d98c5e04