Show N/A when camera doesn't support PTZ in media-internals

This CL makes sure user sees something in the "Pan-Tilt-Zoom" column
when camera doesn't support PTZ in about:media-internals page.

Bug: 934063
Change-Id: Iaabfcaa0da3a2a89e30b7bc33742fba425a4f942
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2445153
Auto-Submit: François Beaufort <beaufort.francois@gmail.com>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813337}

content/browser/resources/media/manager.js

@@ -207,7 +207,7 @@ var Manager = (function() {
                   videoCaptureCapabilities[i]['formats'][j]);
         }
         videoCaptureCapabilities[i]['controlSupport'] =
-            videoCaptureCapabilities[i]['controlSupport'].join(' ');
+            videoCaptureCapabilities[i]['controlSupport'].join(' ') || 'N/A';
       }
 
       // The keys of each device to be shown in order of appearance.