-webkit-text-size-adjust doesn't work correctly on iPad

[Haowen]

Created attachment 459778 [details]
How it looks like on iPad

`configuration.defaultWebpagePreferences.preferredContentMode` has been set to `.mobile`
But “webkit-text-size-adjust” doesn't work correctly on iPad. 
On iPhone, set “webkit-text-size-adjust” with 200%, both font-size and line-height will be scaled.
But on iPad, only font-size gets scaled, line-height doesn't change.

* STEPS TO REPRODUCE
1. Create a WKWebView with `config.defaultWebpagePreferences.preferredContentMode = .mobile`
2. loadHTMLString in WebView with "<h1 style="-webkit-text-size-adjust: 200%; font-size: 100px; line-height: 100px">Hello, World!Hello, World!Hello, World!Hello, World!Hello, World!Hello, World!Hello, World!Hello, World!</h1>"
3. Run the app on iPhone and iPad

* RESULTS

Correct: On iPhone, the font-size and the line-height are all scaled to 200px

Incorrect: On iPad, the font-size is scaled but the line-height is still 100px

[Radar WebKit Bug Importer]

<rdar://problem/94274628>