Add MIPS SIMD Arch (MSA) optimized MirrorRow function

docs/getting_started.md

 # Getting Started
 
 How to get and build the libyuv code.
 
 ## Pre-requisites
 
 You'll need to have depot tools installed: https://www.chromium.org/developers/how-tos/install-depot-tools
 Refer to chromium instructions for each platform for other prerequisites.
 
 ## Getting the Code
@@ skipping 177 matching lines @@
     gn gen out/Release "--args=is_debug=false target_cpu=\"x86\""
     gn gen out/Debug "--args=is_debug=true target_cpu=\"x86\""
     ninja -C out/Release
     ninja -C out/Debug
 
 ### Building Offical with GN
 
     gn gen out/Official "--args=is_debug=false is_official_build=true is_chrome_branded=true"
     ninja -C out/Official
 
+#### Building mips with GN
+
+mipsel
+    gn gen out/Default "--args=is_debug=false target_cpu=\"mipsel\" target_os = \"android\" mips_arch_variant = \"r6\" mips_use_msa = true is_component_build = true is_clang = false"
+    ninja -C out/Default
+
+mips64el
+    gn gen out/Default "--args=is_debug=false target_cpu=\"mips64el\" target_os = \"android\" mips_arch_variant = \"r6\" mips_use_msa = true is_component_build = true is_clang = false"
+    ninja -C out/Default
+
 ### Linux
 
     GYP_DEFINES="target_arch=x64" ./gyp_libyuv
     ninja -j7 -C out/Debug
     ninja -j7 -C out/Release
 
     GYP_DEFINES="target_arch=ia32" ./gyp_libyuv
     ninja -j7 -C out/Debug
     ninja -j7 -C out/Release
 
@@ skipping 213 matching lines @@
     git checkout -b mycl -t origin/master
     git pull
     <edit files>
     git add -u
     git commit -m "my change"
     git cl lint
     git cl try
     git cl upload -r a-reviewer@chomium.org -s
     <once approved..>
     git cl land