[GStreamer] compress property for the HTTP source element

[GStreamer] compress property for the HTTP source element
https://bugs.webkit.org/show_bug.cgi?id=143518

Reviewed by Carlos Garcia Campos.

Added a compress property so the default behavior or not
requesting content encoded to the server can be overridden if
needed. This is useful for adaptive streaming playback.

* platform/graphics/gstreamer/WebKitWebSourceGStreamer.cpp:
(webKitWebSrcSetProperty):
(webKitWebSrcGetProperty):
(webKitWebSrcStart):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@182534 268f45cc-cd09-0410-ab3c-d52691b4dbfc