Make gyp target_arch setting native to chromium/config.py

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

 {
   "blink": {
     "cache_dir": null, 
     "solutions": [
       {
         "custom_deps": {
           "src/third_party/WebKit": "https://src.chromium.org/blink/trunk"
         }, 
         "custom_vars": {
           "webkit_revision": "HEAD"
@@ skipping 118 matching lines @@
     "cache_dir": null, 
     "solutions": [
       {
         "deps_file": "", 
         "managed": true, 
         "name": "src", 
         "url": "https://src.chromium.org/chrome/trunk/src"
       }
     ]
   }, 
+  "libvpx": {
+    "cache_dir": null, 
+    "solutions": [
+      {
+        "custom_deps": {
+          "src/third_party/libvpx/source2/libvpx": "https://chromium.googlesource.com/webm/libvpx.git"
+        }, 
+        "deps_file": "DEPS", 
+        "managed": true, 
+        "name": "src", 
+        "url": "https://src.chromium.org/chrome/trunk/src"
+      }
+    ]
+  }, 
   "nacl": {
     "cache_dir": null, 
     "solutions": [
       {
         "deps_file": "DEPS", 
         "managed": true, 
         "name": "native_client", 
         "url": "https://src.chromium.org/native_client/trunk/src/native_client"
       }, 
       {
@@ skipping 69 matching lines @@
     "solutions": [
       {
         "deps_file": "DEPS", 
         "managed": true, 
         "name": "v8", 
         "url": "http://v8.googlecode.com/svn/branches/bleeding_edge"
       }
     ]
   }
 }