Skia side RGB to YUV gpu conversion

This code is the one that's currently working in my local chromium build. A few things still need to be addressed and I'll highlight these directly in the code.

BUG=skia:
R=reed@google.com, bsalomon@google.com, senorblanco@google.com, senorblanco@chromium.org, robertphillips@google.com, scroggo@google.com, halcanary@google.com

Author: sugoi@chromium.org

Review URL: https://codereview.chromium.org/374743003
diff --git a/gyp/tests.gypi b/gyp/tests.gypi
index 6a64479..34e8f54 100644
--- a/gyp/tests.gypi
+++ b/gyp/tests.gypi
@@ -103,6 +103,7 @@
     '../tests/ImageCacheTest.cpp',
     '../tests/ImageDecodingTest.cpp',
     '../tests/ImageFilterTest.cpp',
+    '../tests/ImageGeneratorTest.cpp',
     '../tests/ImageNewShaderTest.cpp',
     '../tests/InfRectTest.cpp',
     '../tests/InterpolatorTest.cpp',