Mark mixed sandbox as macOS/Windows only

docs/api/app.md

@@ -684,8 +684,8 @@ allowing multiple instances of your app to run, this will ensure that only a
 single instance of your app is running, and other instances signal this
 instance and exit.
 
-`callback` will be called by the first instance with `callback(argv, workingDirectory)` 
-when a second instance has been executed. `argv` is an Array of the second instance's 
+`callback` will be called by the first instance with `callback(argv, workingDirectory)`
+when a second instance has been executed. `argv` is an Array of the second instance's
 command line arguments, and `workingDirectory` is its current working directory. Usually
 applications respond to this by making their primary window focused and
 non-minimized.
@@ -913,7 +913,7 @@ correctly.
 
 **Note:** This will not affect `process.argv`.
 
-### `app.enableMixedSandbox()` _Experimental_
+### `app.enableMixedSandbox()` _Experimental_ _macOS_ _Windows_
 
 Enables mixed sandbox mode on the app.
 

docs/api/sandbox-option.md

@@ -52,7 +52,7 @@ app.on('ready', () => {
 ```
 
 In the above code the `BrowserWindow` that was created has node.js disabled and can communicate
-only via IPC. The use of this option stops electron from creating a node.js runtime in the renderer. Also, 
+only via IPC. The use of this option stops electron from creating a node.js runtime in the renderer. Also,
 within this new window `window.open` follows the native behaviour (by default electron creates a `BrowserWindow`
 and returns a proxy to this via `window.open`).
 
@@ -62,6 +62,7 @@ force `sandbox: true` for all `BrowserWindow` instances.
 
 To enable OS-enforced sandbox on `BrowserWindow` or `webview` process with `sandbox:true` without causing
 entire app to be in sandbox, `--enable-mixed-sandbox` command-line argument must be passed to electron.
+This option is currently only supported on macOS and Windows.
 
 ```js
 let win

spec/api-app-spec.js

@@ -568,6 +568,9 @@ describe('app module', function () {
   })
 
   describe('mixed sandbox option', function () {
+    // FIXME Get these specs running on Linux
+    if (process.platform === 'linux') return
+
     let appProcess = null
     let server = null
     const socketPath = process.platform === 'win32' ? '\\\\.\\pipe\\electron-mixed-sandbox' : '/tmp/electron-mixed-sandbox'