Bug 1873405: Relax an assertion about timing r=sefeng

When RFP is enabled and you're dealing with timestamps of
reduced precision, it's possible for these timestamps to
be equal. Relax the assertion to allow that.

Also, conceptually, it should be <= even without RFP because
you can't guarantee the OS timestamps are going to be reliable
(just increasing) and you can't guarantee anything about how
fast or slow computation is going to happen.

Differential Revision: https://phabricator.services.mozilla.com/D197946

dom/performance/PerformanceMainThread.cpp

@@ -664,7 +664,7 @@ void PerformanceMainThread::ProcessElementTiming() {
       continue;
     }
 
-    MOZ_ASSERT(imagePendingRendering.mLoadTime < rawNowTime);
+    MOZ_ASSERT(imagePendingRendering.mLoadTime <= rawNowTime);
     if (imgRequestProxy* requestProxy =
             imagePendingRendering.GetImgRequestProxy()) {
       LCPHelpers::CreateLCPEntryForImage(