Add MIPS SIMD Arch (MSA) optimized MirrorRow function

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index fed8a68c0d58b15c02df72450e0193997143de3c..f9e913c0d6c4a41fec4ba006a691ee2caa963ed7 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -7,6 +7,7 @@
 # be found in the AUTHORS file in the root of the source tree.
 
 import("//build/config/arm.gni")
+import("//build/config/mips.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 
 config("libyuv_config") {
@@ -17,6 +18,7 @@ config("libyuv_config") {
 }
 
 use_neon = current_cpu == "arm64" || (current_cpu == "arm" && (arm_use_neon || arm_optionally_use_neon))
+use_msa = (current_cpu == "mips64el" || current_cpu == "mipsel") && mips_use_msa
 
 static_library("libyuv") {
   sources = [
@@ -93,6 +95,10 @@ static_library("libyuv") {
     deps += [ ":libyuv_neon" ]
   }
 
+  if (use_msa) {
+    deps += [ ":libyuv_msa" ]
+  }
+  
   if (is_nacl) {
     # Always enable optimization under NaCl to workaround crbug.com/538243 .
     configs -= [ "//build/config/compiler:default_optimization" ]
@@ -122,3 +128,14 @@ if (use_neon) {
     }
   }
 }
+
+if (use_msa) {
+  static_library("libyuv_msa") {
+    sources = [
+      # MSA Source Files
+      "source/row_msa.cc",
+    ]
+
+    public_configs = [ ":libyuv_config" ]
+  }
+}