Tweak -mssse3 on Mac a little.

Tweak -mssse3 on Mac a little.

Using OTHER_CPLUSPLUSFLAGS instead of OTHER_CFLAGS will append -mssse3 into the
argument list instead of overwriting as the old note warns about.  (So it's
actually there twice now for the files in opts_ssse3, and we can still build if
we remove -mssse3 from common_conditions.gypi.)

We could also just delete this clause entirely given that
common_conditions.gypi sets it anyway.  Which do you think is best?  This code
won't compile unless _someone_ has set -mssse3.  Seems to me the redundancy
helps communicate that and protect against changes in common_conditions.gypi.

BUG=
R=epoger@google.com, bungeman@google.com

Author: mtklein@google.com

Review URL: https://chromiumcodereview.appspot.com/21279005

git-svn-id: http://skia.googlecode.com/svn/trunk@10573 2bbb7eff-a529-9590-31e7-b0007b416f81