[WebView] implement missing functionality in PasteboardJava.cpp

The native Clipboard and Pasteboard classes were refactored in WebKit. A part of the functionality migrated from the former to the latter, in the context of a long-term task of Clipboard class removal.

Prior to the latest WebKit update there was a macros defined in WebCore/dom/Clipboard.h:

// This DOM object now works by calling through to classes in the platform layer.
// Specifically, the class currently named Pasteboard. The legacy style instead
// uses this as an abstract base class.
#define WTF_USE_LEGACY_STYLE_ABSTRACT_CLIPBOARD_CLASS (PLATFORM(IOS) || PLATFORM(QT) || PLATFORM(JAVA))

But it was removed later (http://trac.webkit.org/changeset/154230).

Currently, we have Pasteboard implemented in PasteboardJava.cpp. However, a number of methods is marked as NOT IMPLEMENTED. This leads to a crash that can be reproduced as follows:

- Open, for instance, yahoo.com in a Browser. Type any text in the search field, select it and try to drag out. A crash fill follow. Same way, try to select any text on the page and drag & drop it into the search field. Same crash.

Interestingly that the crash is not reproduced on OSX, but on Windows and Linux. A stack dump is attached.