content: Change log channel to 'ContentHandler'

Follows-up 30e54b3962, which accidentally passed the string
'MediaWiki\Content\ContentHandlerFactory::getContentHandler' as
the log channel. We generally use wide and generic channel names
named after a whole component. Or, if using wfDebug(), we
sometimes prefix the method name to the message. It looks like
these two idioms were accidentally mixed up which means this
message is missing when looking for 'ContentHandler', but it
also meant that when viewing the full debug log in Kibana, that
the channel column breaks due to this being too long.

Change-Id: If578893f568905ac0d560ecf544fcc28dd61ba91

includes/content/ContentHandlerFactory.php

@@ -81,7 +81,7 @@ final class ContentHandlerFactory implements IContentHandlerFactory {
 		if ( empty( $this->handlersByModel[$modelID] ) ) {
 			$contentHandler = $this->createForModelID( $modelID );
 
-			wfDebugLog( __METHOD__,
+			wfDebugLog( 'ContentHandler',
 				"Registered handler for {$modelID}: " . get_class( $contentHandler ) );
 			$this->handlersByModel[$modelID] = $contentHandler;
 		}