Bug 1285419 - Telemetry probe: VIDEO_HIDDEN_PLAY_TIME_MS - r=bsmedberg,kamidphish

This probe is similar to VIDEO_PLAY_TIME_MS, but it will only record the total
play time while the video is hidden.

MozReview-Commit-ID: CM62u7EalRY

--HG--
extra : rebase_source : f5305390fb82815648b33fdad11da0f0b2eac0e9

toolkit/components/telemetry/Histograms.json

@@ -8230,6 +8230,15 @@
     "n_buckets": 100,
     "bug_numbers": [1261955, 1127646]
   },
+  "VIDEO_HIDDEN_PLAY_TIME_MS" : {
+    "alert_emails": ["ajones@mozilla.com", "gsquelart@mozilla.com"],
+    "expires_in_version": "55",
+    "description": "Total time spent playing video while element is hidden, in milliseconds. This reports the total hidden play time for an HTML Media Element whenever it is suspended or resumed, such as when the page is unloaded, or when the mute status changes when the AudioChannelAPI pref is set.",
+    "kind": "exponential",
+    "high": 7200000,
+    "n_buckets": 100,
+    "bug_numbers": [1285419]
+  },
   "VIDEO_UNLOAD_STATE": {
     "alert_emails": ["ajones@mozilla.com"],
     "expires_in_version": "55",