Changing all PSNR/SSIM calculations to use libyuv.

Removed old PSNR/SSIM implementations in:
* test/testsupport/metrics/video_metrics.cc
* src/modules/video_coding/codecs/test_framework/test.cc
The functions in video_metrics.cc is now using code in libyuv instead. Old code in test.cc is using the same functions.
The code for video_metrics.h had to be moved into a separate GYP file to avoid circular dependency error on Mac (see issue 160 for more details). The reason for this is that libyuv's unittest target depends on test_support_main.

BUG=
TEST=metrics_unittests in Debug+Release on Linux, Mac and Windows.

Review URL: http://webrtc-codereview.appspot.com/333025

git-svn-id: http://webrtc.googlecode.com/svn/trunk@1325 4adac7df-926f-26a2-2b94-8c16560cd09d

src/modules/video_coding/main/source/video_coding_test.gypi

@@ -13,6 +13,7 @@
       'dependencies': [
          '<(webrtc_root)/../testing/gtest.gyp:gtest',
          '<(webrtc_root)/../test/test.gyp:test_support',
+         '<(webrtc_root)/../test/metrics.gyp:metrics',
          'webrtc_video_coding',
          'rtp_rtcp',
          'webrtc_utility',