[GStreamer] More leaks in TextCombinerGStreamer

[GStreamer] More leaks in TextCombinerGStreamer
https://bugs.webkit.org/show_bug.cgi?id=180331

Reviewed by Michael Catanzaro.

I've noticed more leaks reading the code, but I couldn't check them with valgrind, because I don't know how to
trigger that code path. This patch cleans up the file using smart pointer everywhere.

* platform/graphics/gstreamer/TextCombinerGStreamer.cpp:
(webkit_text_combiner_pad_init):
(webkitTextCombinerPadEvent):
(webkitTextCombinerReleasePad):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@225473 268f45cc-cd09-0410-ab3c-d52691b4dbfc