Bug 1442068 - Add diagnostic probe for tab switch spinners r=liuche

We currently have a few probes which measure how long tab switch
spinners are displayed, but we have little information into their
source. In diagnosing and attempting to lower the number of
spinners that users see, we could use more information about
their source to help prioritize and narrow in on regions of code.

MozReview-Commit-ID: Cw4ejOM9ZSl

--HG--
extra : rebase_source : c44307ab1195eaeda43e10302e10756b366a2740

toolkit/components/telemetry/Histograms.json

@@ -5961,6 +5961,16 @@
     "releaseChannelCollection": "opt-out",
     "description": "Firefox: If the spinner interstitial displays during tab switching, records the time in ms the graphic is visible. This probe is similar to FX_TAB_SWITCH_SPINNER_VISIBLE_MS, but is for truly degenerate cases."
   },
+  "FX_TAB_SWITCH_SPINNER_VISIBLE_TRIGGER": {
+    "record_in_processes": ["main"],
+    "alert_emails": ["dothayer@mozilla.com"],
+    "bug_numbers": [1442068],
+    "expires_in_version": "63",
+    "kind": "categorical",
+    "labels": ["none", "preActions", "postActions", "onLoadTimeout", "onLayersReady", "onSizeModeOrOcc", "onEndSwapDocShells"],
+    "releaseChannelCollection": "opt-out",
+    "description": "Diagnostic probe to aid in categorizing tab switch spinners. Records what most recently set the loadTimer to null if a spinner was displayed."
+  },
   "FX_TAB_SWITCH_REQUEST_TAB_WARMING_STATE": {
     "record_in_processes": ["main"],
     "expires_in_version": "63",