platforms: put context for extension in platform extension class

app/scripts/background.js

@@ -20,7 +20,6 @@ window.log = log
 log.setDefaultLevel(METAMASK_DEBUG ? 'debug' : 'warn')
 
 const platform = new ExtensionPlatform()
-platform.context = 'extension'
 const notificationManager = new NotificationManager()
 global.METAMASK_NOTIFIER = notificationManager
 

app/scripts/metamask-controller.js

@@ -196,7 +196,7 @@ module.exports = class MetamaskController extends EventEmitter {
       },
       // rpc data source
       rpcUrl: this.networkController.getCurrentRpcAddress(),
-      originHttpHeaderKey: this.platform.context === 'extension' ? 'X-Metamask-Origin' : undefined,
+      originHttpHeaderKey: this.platform.isExtension ? 'X-Metamask-Origin' : undefined,
       // account mgmt
       getAccounts: (cb) => {
         const isUnlocked = this.keyringController.memStore.getState().isUnlocked

app/scripts/platforms/extension.js

@@ -5,6 +5,9 @@ class ExtensionPlatform {
   //
   // Public
   //
+  get isExtension () {
+    return true
+  }
 
   reload () {
     extension.runtime.reload()