Add Gamepad logging channel

Add Gamepad logging channel
https://bugs.webkit.org/show_bug.cgi?id=90570

Reviewed by Carlos Garcia Campos.

Add a new logging channel for logging significant
events that are related to Gamepad API implementation.

No new tests - no new testable functionality.

* platform/Logging.cpp:
(WebCore):
(WebCore::getChannelFromName):
* platform/Logging.h:
(WebCore):
* platform/gtk/GamepadsGtk.cpp: Log when a device is
registered or unregistered.
(WebCore::GamepadsGtk::registerDevice):
(WebCore::GamepadsGtk::unregisterDevice):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121895 268f45cc-cd09-0410-ab3c-d52691b4dbfc