BUILD FIX (r143637): Predeclare JSContext as an Objective-C class
<http://webkit.org/b/106059>

Fixes the following build failure:

    In file included from Source/WebKit/ios/DefaultDelegates/WebDefaultFrameLoadDelegate.m:11:
    In file included from Source/WebKit/mac/WebView/WebFrameLoadDelegatePrivate.h:29:
    Source/WebKit/mac/WebView/WebFrameLoadDelegate.h:215:64: error: expected a type
    - (void)webView:(WebView *)webView didCreateJavaScriptContext:(JSContext *)context forFrame:(WebFrame *)frame;
                                                                   ^
    1 error generated.

* WebView/WebFrameLoadDelegate.h: Add @class JSContext
declaration.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144545 268f45cc-cd09-0410-ab3c-d52691b4dbfc