Enable mute button, even if preferring hidden volume slider.

LayoutTests/media/video-controls-hidden-audio.html

 <video controls></video>
 <p>Test that hiding volume / mute buttons works as expected.</p>
 <script src=media-file.js></script>
 <script src=video-test.js></script>
 <script src=media-controls.js></script>
 <script>
     video.src = findMediaFile("video", "content/test");
     run("video.load()");
     waitForEvent("canplaythrough", function () {
         // Enable hidden audio preferences to take effect.
         run("window.internals.setAllowHiddenVolumeControls(video, true)");
-        // Request non-hidden audio.
+        // Request non-hidden audio controls.
         run("window.internals.settings.setPreferHiddenVolumeControls(false)");
         run("video.muted = false");
         muteButton = mediaControlsButton(video, "mute-button");
         volumeSlider = mediaControlsButton(video, "volume-slider");
 
         // Make sure that both are visible.
         testExpected("getComputedStyle(muteButton).display", "none", '!=');
         testExpected("getComputedStyle(volumeSlider).display", "none", '!=');
 
-        // Switch to muted video.  Both should still be visible.  We then
-        // remind it that we'd like to allow hidden audio preferences, since
-        // that also resets some state.
+        // Switch to muted video.  Both should still be visible.
         run("video.muted = true");
-        run("window.internals.setAllowHiddenVolumeControls(video, true)");
         testExpected("getComputedStyle(muteButton).display", "none", '!=');
         testExpected("getComputedStyle(volumeSlider).display", "none", '!=');
 
         run("window.internals.settings.setPreferHiddenVolumeControls(true)");
 
         // Switch back to unmuted video.
         run("video.muted = false");
-        run("window.internals.setAllowHiddenVolumeControls(video, true)");
 
-        testExpected("getComputedStyle(muteButton).display", "none", '==');
+        testExpected("getComputedStyle(muteButton).display", "none", '!=');
         testExpected("getComputedStyle(volumeSlider).display", "none", '==');
 
         // For muted video, the volume slider will hide but the mute
-        // button should come back, to let the user unmute.
+        // button should stay, since we always have it present for media
+        // which have audio.
         run("video.muted = true");
-        run("window.internals.setAllowHiddenVolumeControls(video, true)");
         testExpected("getComputedStyle(muteButton).display", "none", '!=');
         testExpected("getComputedStyle(volumeSlider).display", "none", '==');
 
         endTest();
     });
 </script>