color: Clarify default behaviors

ui/gfx/color_transform.cc

Index: ui/gfx/color_transform.cc
diff --git a/ui/gfx/color_transform.cc b/ui/gfx/color_transform.cc
index 49a3026c895e13c33926b613c6724dc4e45d723c..e427ad709526b11b6855c0c583667b62dabd9ac1 100644
--- a/ui/gfx/color_transform.cc
+++ b/ui/gfx/color_transform.cc
@@ -686,6 +686,11 @@ ScopedQcmsProfile GetXYZD50Profile() {
 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;