Bug 1535223 - part1 : rename log module's name. r=jya

Use more general name `WebVTT` for this log module, which will include other debug logs in other files later.

Differential Revision: https://phabricator.services.mozilla.com/D23447

--HG--
extra : moz-landing-system : lando

dom/html/TextTrackManager.cpp

@@ -21,7 +21,7 @@
 #include "nsVariant.h"
 #include "nsVideoFrame.h"
 
-static mozilla::LazyLogModule gTextTrackLog("TextTrackManager");
+static mozilla::LazyLogModule gTextTrackLog("WebVTT");
 #define WEBVTT_LOG(...) MOZ_LOG(gTextTrackLog, LogLevel::Debug, (__VA_ARGS__))
 #define WEBVTT_LOGV(...) \
   MOZ_LOG(gTextTrackLog, LogLevel::Verbose, (__VA_ARGS__))