color: Clarify default behaviors

cc/output/gl_renderer.cc

Index: cc/output/gl_renderer.cc
diff --git a/cc/output/gl_renderer.cc b/cc/output/gl_renderer.cc
index f9cc3d71c3a232ca1d4b6aef52354891ed7e18c8..8948286f202d243089b7c64d9f06dabaa460b2e6 100644
--- a/cc/output/gl_renderer.cc
+++ b/cc/output/gl_renderer.cc
@@ -2108,6 +2108,9 @@ void GLRenderer::DrawYUVVideoQuad(const YUVVideoDrawQuad* quad,
         break;
     }
   }
+  // Invalid or unspecified color spaces should be treated as REC709.
+  if (!src_color_space.IsValid())
+    src_color_space = gfx::ColorSpace::CreateREC709();
 
   ResourceProvider::ScopedSamplerGL y_plane_lock(
       resource_provider_, quad->y_plane_resource_id(), GL_TEXTURE1, GL_LINEAR);