Updated WebView.mm with the latest code.
Fixed up memory management codes.

For non-ARC projects. Unity doesn't use ARC for iOS builds so this is good for them.
Merged WebViewObject.cs with the latest code.

from D&G 3.0 dev
Refactored codes.
Implemented ReloadURL for Mac OS X
Implemented SetCenterPositionWithScale for OS X.
Fixed a bug of SetCenterPositionWithScale in Editor.