Bug 1373888 - part3 : rename the topic of the wake lock. r=cpearce

For knowing the wake lock usage more clearly, we should use more specific topic name.

In OSX, you can use "$ pmset -g assertions" to check all the wakelock.
In Windows, using "$ powser -energy" to generate the energy report.

MozReview-Commit-ID: rAXnkxTvLc

dom/html/HTMLVideoElement.cpp

@@ -334,7 +334,7 @@ HTMLVideoElement::UpdateScreenWakeLock()
     NS_ENSURE_TRUE_VOID(pmService);
 
     ErrorResult rv;
-    mScreenWakeLock = pmService->NewWakeLock(NS_LITERAL_STRING("screen"),
+    mScreenWakeLock = pmService->NewWakeLock(NS_LITERAL_STRING("video-playing"),
                                              OwnerDoc()->GetInnerWindow(),
                                              rv);
   }