color: Clarify default behaviors

ui/gfx/color_transform.cc

 // Copyright (c) 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/gfx/color_transform.h"
 
 #include <algorithm>
 #include <cmath>
 #include <list>
 #include <memory>
@@ skipping 668 matching lines @@
   xyz.blue.Y = 1.0f;
   w.x = 0.34567f;
   w.y = 0.35850f;
   w.Y = 1.0f;
   return ScopedQcmsProfile(qcms_profile_create_rgb_with_gamma(w, xyz, 1.0f));
 }
 
 ColorTransformInternal::ColorTransformInternal(const ColorSpace& from,
                                                const ColorSpace& to,
                                                Intent intent) {
+  // If no source color space is specified, do no transformation.
+  // TODO(ccameron): We may want to assume sRGB at some point in the future.
+  if (!from.IsValid())
+    return;
+
   ScopedQcmsProfile from_profile = GetQCMSProfileIfAvailable(from);
   ScopedQcmsProfile to_profile = GetQCMSProfileIfAvailable(to);
   bool has_from_profile = !!from_profile;
   bool has_to_profile = !!to_profile;
 
   if (from_profile) {
     steps_.push_back(base::MakeUnique<QCMSColorTransform>(
         std::move(from_profile), GetXYZD50Profile()));
   }
 
@@ skipping 49 matching lines @@
     const ColorSpace& to,
     Intent intent) {
   return std::unique_ptr<ColorTransform>(
       new ColorTransformInternal(from, to, intent));
 }
 
 ColorTransform::ColorTransform() {}
 ColorTransform::~ColorTransform() {}
 
 }  // namespace gfx