Have V8Clipboard::clearDataMethodCustom() use V8TRYCATCH_FOR_V8STRINGRESOURCE_VOID()

Have V8Clipboard::clearDataMethodCustom() use V8TRYCATCH_FOR_V8STRINGRESOURCE_VOID() macro
instead of toWebCoreString() so that ClipboardChromium::clearData() is not called if
the string conversion failed.

Also remove a wrong ASSERT_NOT_REACHED() from ClipboardChromium::clearData() that was
causing us to crash whenever this method was called in a debug build.
ClipboardData.clearData() was not covered by layout tests until now which is why we
did not notice it before.

TEST=fast/events/clipboard-dataTransferItemList.html
R=haraken
BUG=

Review URL: https://chromiumcodereview.appspot.com/23496045

git-svn-id: svn://svn.chromium.org/blink/trunk@157601 bbb929c8-8fbe-4397-9dbb-9b2b20218538