Use RgbaColor in DesktopFrameGenerator and add RgbaColorTest

webrtc/modules/desktop_capture/rgba_color.cc

Index: webrtc/modules/desktop_capture/rgba_color.cc
diff --git a/webrtc/modules/desktop_capture/rgba_color.cc b/webrtc/modules/desktop_capture/rgba_color.cc
index 2342b46915061a62926149c8464f7ed0590eec44..89212ef3eb5f9d041273265dfe078ca8e680a70a 100644
--- a/webrtc/modules/desktop_capture/rgba_color.cc
+++ b/webrtc/modules/desktop_capture/rgba_color.cc
@@ -36,6 +36,9 @@ RgbaColor::RgbaColor(uint8_t blue, uint8_t green, uint8_t red)
 RgbaColor::RgbaColor(const uint8_t* bgra)
     : RgbaColor(bgra[0], bgra[1], bgra[2], bgra[3]) {}
 
+RgbaColor::RgbaColor(uint32_t bgra)
+    : RgbaColor(reinterpret_cast<uint8_t*>(&bgra)) {}
+
 bool RgbaColor::operator==(const RgbaColor& right) const {
   return blue == right.blue && green == right.green && red == right.red &&
          AlphaEquals(alpha, right.alpha);
@@ -45,4 +48,8 @@ bool RgbaColor::operator!=(const RgbaColor& right) const {
   return !(*this == right);
 }
 
+uint32_t RgbaColor::ToUInt32() const {
+  return *(reinterpret_cast<const uint32_t*>(&blue));

[Sergey Ulanov, 2016/09/13 00:47:48]

I don't think you can use reinterpret_cast<> here without making assumption
about how the struct is packed in memory.
I think you need something like (alpha<<24 | red << 16 | green << 8 | blue)

[Hzj_jie, 2016/09/13 18:48:10]

I think the endian will be broken by using shift, since this uint32 should be
able to directly assign to or from DesktopFrame::data(), which is always
little-endian. Have I made a mistake here?

[Sergey Ulanov, 2016/09/19 20:35:28]

All platforms we currently support are little-endian (or bi-endian used in
little-endian mode): see
https://codesearch.chromium.org/chromium/src/build/build_config.h?rcl=0&l=104
I don't know which layout we will want to use for data in DesktopFrame if a
big-endian arch support ever is added in the future (which is by itself is very
unlikely), so I don't think we need to worry about it, but you can add ifdef
ARCH_CPU_LITTLE_ENDIAN if you are concerned about it and mark the big-endian
case as not implemented?

[Hzj_jie, 2016/09/20 00:00:08]

Done.

+}
+
 }  // namespace webrtc