DevTools: fix npm_test architecture check

process.arch is based on the architecture which node.js was compiled for,
not necessarily the architecture of the system the node.js process is
currently running in.

I came across this issue while working on buildbot related stuff.

More info:
nodejs/node-v0.x-archive#2862

BUG=none

Review-Url: https://codereview.chromium.org/2698983004
Cr-Commit-Position: refs/heads/master@{#451511}

Author: wwwillchen

third_party/WebKit/Source/devtools/scripts/npm_test.js

@@ -145,14 +145,10 @@ function copyFrontend(contentShellResourcesPath) {
 
 function getPlatform() {
   if (process.platform === 'linux') {
-    if (process.arch === 'x64')
-      return 'Linux_x64';
-    throw new Error('Pre-compiled content shells are only available for x64 on Linux');
+    return 'Linux_x64';
   }
   if (process.platform === 'win32') {
-    if (process.arch === 'x64')
-      return 'Win_x64';
-    return 'Win';
+    return 'Win_x64';
   }
   if (process.platform === 'darwin')
     return 'Mac';