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/HTMLMediaElement.cpp

@@ -4189,7 +4189,7 @@ HTMLMediaElement::WakeLockCreate()
     NS_ENSURE_TRUE_VOID(pmService);
 
     ErrorResult rv;
-    mWakeLock = pmService->NewWakeLock(NS_LITERAL_STRING("cpu"),
+    mWakeLock = pmService->NewWakeLock(NS_LITERAL_STRING("audio-playing"),
                                        OwnerDoc()->GetInnerWindow(),
                                        rv);
   }