Use RgbaColor in DesktopFrameGenerator and add RgbaColorTest

webrtc/modules/desktop_capture/rgba_color.h

 /*
  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 16 matching lines @@
 struct RgbaColor final {
   // Creates a color with BGRA channels.
   RgbaColor(uint8_t blue, uint8_t green, uint8_t red, uint8_t alpha);
 
   // Creates a color with BGR channels, and set alpha channel to 255 (opaque).
   RgbaColor(uint8_t blue, uint8_t green, uint8_t red);
 
   // Creates a color from four-byte in BGRA order, i.e. DesktopFrame::data().
   explicit RgbaColor(const uint8_t* bgra);
 
+  // Creates a color from BGRA channels in a uint format. Consumers should make
+  // sure the memory order of the uint32_t is always BGRA from left to right, no
+  // matter the system endian. This function creates an equivalent RgbaColor
+  // instance from the ToUInt32() result of another RgbaColor instance.
+  explicit RgbaColor(uint32_t bgra);
+
   // Returns true if |this| and |right| is the same color.
   bool operator==(const RgbaColor& right) const;
 
   // Returns true if |this| and |right| are different colors.
   bool operator!=(const RgbaColor& right) const;
 
+  uint32_t ToUInt32() const;
+
   uint8_t blue;
   uint8_t green;
   uint8_t red;
   uint8_t alpha;
 };
 static_assert(
     DesktopFrame::kBytesPerPixel == sizeof(RgbaColor),
     "A pixel in DesktopFrame should be safe to be represented by a RgbaColor");
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_DESKTOP_CAPTURE_RGBA_COLOR_H_