Transition ScratchJr-iOS to use WKWebview

[chrisgarrity]

From @tmickel on February 3, 2016 16:58

Currently we're using UIWebview. WKWebview is supported on iOS 8+, which according to our analytics is at least 80% of iOS users. Switching to WKWebview will provide better performance, a much nicer JS-native interface, and hopefully less bugs.

Copied from original issue: LLK/scratchjr-private#466

[yueyuzhao]

With WKWebView we can't get a return value directly from native which differs from Android.

[yueyuzhao]

ScratchJr UIWebView memory usage:

ScratchJr WKWebView memory usage:

[chrisgarrity]

Fixed in https://github.com/LLK/scratchjr/pull/291