BUG=319
TESTED=libyuv_unittest
R=fbarchard@google.com

Add the following functions:
- ScaleRowDown38_NEON
- ScaleRowDown38_2_Box_NEON
- ScaleRowDown38_3_Box_NEON

I find that these functions aren't tracked in the gtest.
So that I write the test case myself.

Change-Id: Ie70a00d7f708450dc786dfb388386ff748a21508

Review URL: https://webrtc-codereview.appspot.com/15229004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1071 16f28f9a-4ce2-e073-06de-1de4eb20be90