Bug 1416044 - Add telemetry probe for if userChrome.css is loaded. r=francois,jwatt

* New USER_CHROME_CSS_LOADED histogram
* Just log a boolean to reflect if the userChrome.css was successfully loaded at profile initialization, in the parent process.

MozReview-Commit-ID: 8ffSNUot43I

--HG--
extra : rebase_source : 11f43e832449952c83cdb104c61c9c33c1aaa123

layout/style/nsLayoutStylesheetCache.cpp

@@ -13,6 +13,7 @@
 #include "mozilla/Preferences.h"
 #include "mozilla/StyleSheet.h"
 #include "mozilla/StyleSheetInlines.h"
+#include "mozilla/Telemetry.h"
 #include "mozilla/css/Loader.h"
 #include "mozilla/dom/SRIMetadata.h"
 #include "MainThreadUtils.h"
@@ -441,6 +442,12 @@ nsLayoutStylesheetCache::InitFromProfile()
 
   LoadSheetFile(contentFile, &mUserContentSheet, eUserSheetFeatures, eLogToConsole);
   LoadSheetFile(chromeFile, &mUserChromeSheet, eUserSheetFeatures, eLogToConsole);
+
+  if (XRE_IsParentProcess()) {
+    // We're interested specifically in potential chrome customizations,
+    // so we only need data points from the parent process
+    Telemetry::Accumulate(Telemetry::USER_CHROME_CSS_LOADED, mUserChromeSheet != nullptr);
+  }
 }
 
 void

toolkit/components/telemetry/Histograms.json

@@ -13844,6 +13844,15 @@
     "n_values": 64,
     "description": "URL CLassifier-related (aka Safe Browsing) UI events. See nsIUrlClassifierUITelemetry.idl for the specific values."
   },
+  "USER_CHROME_CSS_LOADED": {
+    "record_in_processes": ["main"],
+    "bug_numbers": [1416044],
+    "expires_in_version": "62",
+    "alert_emails": ["sfoster@mozilla.com"],
+    "kind": "boolean",
+    "releaseChannelCollection": "opt-out",
+    "description": "Was a userChrome.css stylesheet loaded when the profile was initialized?"
+  },
  "WEBAUTHN_CREATE_CREDENTIAL_MS": {
     "record_in_processes": ["main"],
     "alert_emails": ["seceng-telemetry@mozilla.com", "jjones@mozilla.com"],