🏁 Preserve file extension when extracting from asar

Currently, when calling `copyFileOut`, the original extension from the
file is lost, and a generic `*.tmp` is added instead.

This becomes problematic in the scenario where we use
`child_process.execFile` on a Windows Batch script that lives inside the
`asar` package.

Windows relies on the extension being present in order to interpret the
script accordingly, which results in the following bug because the
operating system doesn't know what do to with this `*.tmp` file:

```
Error: spawn UNKNOWN
```

Steps to reproduce:

1. Create a dummy batch script (test.bat):

```
@echo off
echo "Hello world"
```

2. Create an electron app that attemps to call this script with
`child_process.execFile`:

```js
var child_process = require('child_process');
var path = require('path');

child_process.execFile(path.join(__dirname, 'test.bat'), function(error, stdout) {
    if (error) throw error;
    console.log(stdout);
});
```

3. Package this small application as an asar archive:

```sh
> asar pack mytestapp app.asar
```

4. Execute the application:

```sh
> electron.exe app.asar
```

atom/common/asar/scoped_temporary_file.h

@@ -22,11 +22,13 @@ class ScopedTemporaryFile {
   ScopedTemporaryFile();
   virtual ~ScopedTemporaryFile();
 
-  // Init an empty temporary file.
-  bool Init();
+  // Init an empty temporary file with a certain extension.
+  bool Init(const base::FilePath::StringType ext);
 
   // Init an temporary file and fill it with content of |path|.
-  bool InitFromFile(base::File* src, uint64 offset, uint64 size);
+  bool InitFromFile(base::File* src,
+                    const base::FilePath::StringType ext,
+                    uint64 offset, uint64 size);
 
   base::FilePath path() const { return path_; }