Add color xform support to SkWebpCodec

src/codec/SkCodecPriv.h

 /*
  * Copyright 2015 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkCodecPriv_DEFINED
 #define SkCodecPriv_DEFINED
 
@@ skipping 313 matching lines @@
 
     // Need a color xform when dst space does not match the src.
     bool srcDstNotEqual = !SkColorSpace::Equals(srcInfo.colorSpace(), dstInfo.colorSpace());
 
     // We never perform a color xform in legacy mode.
     bool isLegacy = nullptr == dstInfo.colorSpace();
 
     return !isLegacy && (needsPremul || isF16 || srcDstNotEqual);
 }
 
+static inline SkAlphaType select_alpha_xform(SkAlphaType dstAlphaType, SkAlphaType srcAlphaType) {
+    return (kOpaque_SkAlphaType == srcAlphaType) ? kOpaque_SkAlphaType : dstAlphaType;
+}
+
 #endif // SkCodecPriv_DEFINED