Bug 1664215 - Specify that PDF_VIEWER_FALLBACK_REASON is only collected in content processes r=bdahl,chutten

Differential Revision: https://phabricator.services.mozilla.com/D89782

toolkit/components/telemetry/Histograms.json

@@ -9398,7 +9398,7 @@
     "description": "Time spent to display first page in PDF Viewer (ms)"
   },
   "PDF_VIEWER_FALLBACK_REASON": {
-    "record_in_processes": ["main", "content"],
+    "record_in_processes": ["content"],
     "products": ["firefox"],
     "expires_in_version": "84",
     "alert_emails": ["bdahl@mozilla.com"],