Enable zlib SIMD optimisations on Mac OS X builds

Enable zlib SIMD optimisations on Mac OS X builds

With ninja builds generated from gyp on a Mac it is necessary to specify the
cflags to use as EXTRA_CFLAGS in an xcode_settings block in the gyp file. With
gn this is not necessary and led to confusion resulting in Mac builds not being
enabled in the original CL. As all Intel based Mac systems have SSE2 the
compiler does not require "-msse2" as it can always use that.

This change also initialised the global variables, as a result the variable is
moved from being a common symbol to being in the BSS section of the library.
This is necessary as unfortunately the linker on Mac cannot handle common
symbols with static libraries.

blink layout test inspector/layers/layer-canvas-log.html may start failing on
Mac with this change (See BUG below)

TEST=Build on Mac OS X and net_unittests --gtest_filter=*Compressed* run and
uses optimised code paths
BUG=430557
Committed: https://crrev.com/01686c1a7b233fcb8ab16b41ad3ffe58aef09cd1
Cr-Commit-Position: refs/heads/master@{#304089}

[robert.bradford, 2014-11-07 12:29:03]

robert.bradford@intel.com changed reviewers:
+ agl@chromium.org

[robert.bradford, 2014-11-07 12:29:04]

Hi Adam, I was able to identify and fix the build issue on Mac OS X. It was due
to my misunderstanding the behaviour of gyp on Mac. It uses the EXTRA_CFLAGS
from the xcode_settings block even when generating for ninja whereas gn uses the
generic cflags.

There was also an interesting linking problem with a "component=shared_library"
build where the uninitialized x86_cpu_enable_simd symbol was being categorised
as a "Common Symbol" which prevented linking. By explicitly initialising it to
zero we resolve this linking problem (details: http://goo.gl/kCKySt)

PTAL when you have a moment.

[agl, 2014-11-13 20:29:07]

The CQ bit was checked by agl@chromium.org

[agl, 2014-11-13 20:29:08]

lgtm

Sorry, I missed the email notification for this one.

[commit-bot: I haz the power, 2014-11-13 20:29:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/705053002/20001

[commit-bot: I haz the power, 2014-11-13 21:39:07]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-11-13 21:40:08]

Patchset 2 (id:??) landed as
https://crrev.com/01686c1a7b233fcb8ab16b41ad3ffe58aef09cd1
Cr-Commit-Position: refs/heads/master@{#304089}