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:

Review URL: https://codereview.chromium.org/872793002
1 file changed