Make gyp target_arch setting native to chromium/config.py

scripts/slave/recipe_modules/gclient/config.expected/using_mirror-False-git_mode-True-cache_dir-False.json

 {
   "blink": {
     "cache_dir": null, 
     "solutions": [
       {
         "custom_deps": {
           "src/third_party/WebKit": "https://chromium.googlesource.com/chromium/blink.git"
         }, 
         "custom_vars": {
           "webkit_revision": "HEAD"
@@ skipping 118 matching lines @@
     "cache_dir": null, 
     "solutions": [
       {
         "deps_file": "", 
         "managed": true, 
         "name": "src", 
         "url": "https://chromium.googlesource.com/chromium/src.git"
       }
     ]
   }, 
+  "libvpx": {
+    "cache_dir": null, 
+    "solutions": [
+      {
+        "custom_deps": {
+          "src/third_party/libvpx/source2/libvpx": "https://chromium.googlesource.com/webm/libvpx.git"
+        }, 
+        "deps_file": ".DEPS.git", 
+        "managed": true, 
+        "name": "src", 
+        "url": "https://chromium.googlesource.com/chromium/src.git"
+      }
+    ]
+  }, 
   "nacl": "nacl only supports svn", 
   "oilpan": "Oilpan requires SVN for now", 
   "oilpan_internal": "config \"oilpan_internal\" includes \"oilpan\", but [Oilpan requires SVN for now]", 
   "tools_build": {
     "cache_dir": null, 
     "solutions": [
       {
         "deps_file": ".DEPS.git", 
         "managed": true, 
         "name": "build", 
         "url": "https://chromium.googlesource.com/chromium/tools/build.git"
       }
     ]
   }, 
   "v8": {
     "cache_dir": null, 
     "solutions": [
       {
         "deps_file": ".DEPS.git", 
         "managed": true, 
         "name": "v8", 
         "url": "http://v8.googlecode.com/svn/branches/bleeding_edge"
       }
     ]
   }
 }