If I set the style on any text in a `BrowserWindow` as such:

```
P {
   font-family: cursive;
}
```

...I would expect that text to take on a "cursive" look, much as it does when I apply the same style to a page loaded in Chrome Browser.

However in Electron 1.4.12 on Mac it looks like it's `serif`.

`cursive` is one of the W3C's five standard styles along with `serif`, `sans-serif`, `monospace`, and `fantasy` [1]. All of these work as expected in Electron except `cursive`.

It seems it should work just like the others. I presume it's just an omission somewhere?

Thanks!

[1] https://www.w3.org/wiki/CSS/Properties/font-family
atom/browser/web_contents_preferences.cc
#include "atom/browser/web_contents_preferences.h"
#include <algorithm>
#include <string>
#include <vector>
#include "atom/browser/native_window.h"
#include "atom/browser/web_view_manager.h"
#include "atom/common/native_mate_converters/value_converter.h"
#include "atom/common/options_switches.h"
#include "base/command_line.h"
#include "base/strings/string_number_conversions.h"
#include "cc/base/switches.h"
#include "content/public/browser/render_process_host.h"
#include "content/public/common/content_switches.h"
#include "content/public/common/web_preferences.h"
#include "native_mate/dictionary.h"
#include "net/base/filename_util.h"
#if defined(OS_WIN)
#include "ui/gfx/switches.h"
#endif
DEFINE_WEB_CONTENTS_USER_DATA_KEY(atom::WebContentsPreferences);
namespace atom {
std::vector<WebContentsPreferences*> WebContentsPreferences::instances_;
WebContentsPreferences::WebContentsPreferences(