[Qt] Handle Return key without key text

https://bugs.webkit.org/show_bug.cgi?id=117239

Reviewed by Andreas Kling.

Source/WebCore:

Test added to tst_qwebpage.cpp.

* platform/qt/PlatformKeyboardEventQt.cpp:
(WebCore::keyTextForKeyEvent):

Source/WebKit/qt:

Test for QKeyEvent(Return_Key) without key text.

* tests/qwebpage/tst_qwebpage.cpp:
(tst_QWebPage::inputMethods):

Change-Id: I2e6ee33d90fd740ed2cb17580b1a6fa2f196943f
git-svn-id: http://svn.webkit.org/repository/webkit/trunk@151213 268f45cc-cd09-0410-ab3c-d52691b4dbfc
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>