Bug 1692350 - Implement a telemetry probe to collect page load data for documents that use lazyload r=emilio,chutten

Differential Revision: https://phabricator.services.mozilla.com/D104912

browser/base/content/browser.js

@@ -5776,6 +5776,15 @@ var TabsProgressListener = {
           stopwatchRunning /* we won't see STATE_START events for pre-rendered tabs */
         ) {
           if (recordLoadTelemetry) {
+            if (aBrowser.browsingContext?.topWindowContext?.hadLazyLoadImage) {
+              let timeElapsed = TelemetryStopwatch.timeElapsed(
+                histogram,
+                aBrowser
+              );
+              Services.telemetry
+                .getHistogramById("FX_LAZYLOAD_IMAGE_PAGE_LOAD_MS")
+                .add(timeElapsed);
+            }
             TelemetryStopwatch.finish(histogram, aBrowser);
             BrowserTelemetryUtils.recordSiteOriginTelemetry(browserWindows());
           }