[GTK] UserMedia Permission Request API

[GTK] UserMedia Permission Request API
https://bugs.webkit.org/show_bug.cgi?id=136449

Reviewed by Carlos Garcia Campos.

Source/WebCore:

Very basic constraints validation support in the GStreamer
MediaStreamCenter. This is needed so the GTK C API tests using the
getUserMedia() API would not time out.

* platform/mediastream/gstreamer/MediaStreamCenterGStreamer.cpp:
(WebCore::MediaStreamCenterGStreamer::~MediaStreamCenterGStreamer):
(WebCore::MediaStreamCenterGStreamer::validateRequestConstraints):
(WebCore::MediaStreamCenterGStreamer::createMediaStream):

Source/WebKit2:

* PlatformGTK.cmake: GTK-specific new files.
* UIProcess/API/gtk/WebKitUIClient.cpp:  DecidePolicy handler
implementation for UserMediaPermissionRequest.
* UIProcess/API/gtk/WebKitUserMediaPermissionRequest.cpp: Added,
WebKitGTK API for UserMediaPermissionRequest, very similar to the
one used for Geolocation.
(webkitUserMediaPermissionRequestAllow):
(webkitUserMediaPermissionRequestDeny):
(webkit_permission_request_interface_init):
(webkitUserMediaPermissionRequestDispose):
(webkit_user_media_permission_get_media_types):
(webkitUserMediaPermissionRequestGetProperty):
(webkit_user_media_permission_request_class_init):
(webkitUserMediaPermissionRequestCreate):
* UIProcess/API/gtk/WebKitUserMediaPermissionRequest.h: Added.
* UIProcess/API/gtk/WebKitUserMediaPermissionRequestPrivate.h: Added.
* UIProcess/API/gtk/docs/webkit2gtk-docs.sgml: Expose new API
in documentation.
* UIProcess/API/gtk/docs/webkit2gtk-sections.txt: Ditto.
* UIProcess/API/gtk/webkit2.h: Expose UserMediaPermissionRequest API.

Tools:

The GTK version of the MiniBrowser has gained UserMedia permission
request support.

* MiniBrowser/gtk/BrowserWindow.c: Show a popup when the page
requires user permission to access audio/video devices.
(permissionRequestDialogCallback):
(webViewDecidePermissionRequest):
(geolocationRequestDialogCallback): Deleted.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@176952 268f45cc-cd09-0410-ab3c-d52691b4dbfc