media: Add support for playback of VP8 Alpha video streams

media/base/simd/convert_yuv_to_rgb_x86.cc

Index: media/base/simd/convert_yuv_to_rgb_x86.cc
diff --git a/media/base/simd/convert_yuv_to_rgb_x86.cc b/media/base/simd/convert_yuv_to_rgb_x86.cc
index 37b168d8719ff37c579e84dab7d182bcb1d28af6..1c407d18e3b074270882d8a862cf0a6a24f236fe 100644
--- a/media/base/simd/convert_yuv_to_rgb_x86.cc
+++ b/media/base/simd/convert_yuv_to_rgb_x86.cc
@@ -40,6 +40,37 @@ void ConvertYUVToRGB32_MMX(const uint8* yplane,
   EmptyRegisterState();
 }
 
+void ConvertYUVAToARGB_MMX(const uint8* yplane,
+                           const uint8* uplane,
+                           const uint8* vplane,
+                           const uint8* aplane,
+                           uint8* rgbframe,
+                           int width,
+                           int height,
+                           int ystride,
+                           int uvstride,
+                           int astride,
+                           int rgbstride,
+                           YUVType yuv_type) {
+  unsigned int y_shift = yuv_type;
+  for (int y = 0; y < height; ++y) {
+    uint8* rgb_row = rgbframe + y * rgbstride;
+    const uint8* y_ptr = yplane + y * ystride;
+    const uint8* u_ptr = uplane + (y >> y_shift) * uvstride;
+    const uint8* v_ptr = vplane + (y >> y_shift) * uvstride;
+    const uint8* a_ptr = aplane + y * astride;
+
+    ConvertYUVAToARGBRow_MMX(y_ptr,
+                             u_ptr,
+                             v_ptr,
+                             a_ptr,
+                             rgb_row,
+                             width);
+  }
+
+  _mm_empty();
+}
+
 void ConvertYUVToRGB32_SSE(const uint8* yplane,
                            const uint8* uplane,
                            const uint8* vplane,