Do not report 0 LargestImagePaint and LargestTextPaint to UMA/UKM

This CL is a revert of the functional parts of commit ef514667. We
have decided that these should not be reported as they skew the values
in the histograms in undesirable ways. These 0 values correspond to
times during which the largest image is not yet fully loaded.

Bug: 1017335
Change-Id: I6bbae41f0e0357198f752291c50a439003cd9bc7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1875285Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710544}

components/page_load_metrics/browser/observers/largest_contentful_paint_handler.h

@@ -33,8 +33,6 @@ class ContentfulPaintTimingInfo {
   }
 
   bool IsEmpty() const {
-    // |size_| is not necessarily 0, for example, when the largest image is
-    // still loading.
     return !time_;
   }
 

components/page_load_metrics/renderer/metrics_render_frame_observer.cc

@@ -382,20 +382,24 @@ mojom::PageLoadTimingPtr MetricsRenderFrameObserver::GetTiming() const {
     timing->paint_timing->first_meaningful_paint =
         ClampDelta(perf.FirstMeaningfulPaint(), start);
   }
-  if (perf.LargestImagePaintSize() > 0) {
+  if (perf.LargestImagePaint() > 0.0) {
     timing->paint_timing->largest_image_paint =
-        perf.LargestImagePaint() == 0.0
-            ? base::TimeDelta()
-            : ClampDelta(perf.LargestImagePaint(), start);
+        ClampDelta(perf.LargestImagePaint(), start);
     timing->paint_timing->largest_image_paint_size =
         perf.LargestImagePaintSize();
+    // LargestImagePaintSize should be available if LargestImagePaint is
+    // available. Note that size can be nonzero while the time is 0 since a time
+    // of 0 is sent when the image is painting. We are intentionally ignoring
+    // these cases, as they should not be reported by the UMA/UKM histograms.
+    DCHECK(perf.LargestImagePaintSize());
   }
-  if (perf.LargestTextPaintSize() > 0) {
+  if (perf.LargestTextPaint() > 0.0) {
     timing->paint_timing->largest_text_paint =
-        perf.LargestTextPaint() == 0.0
-            ? base::TimeDelta()
-            : ClampDelta(perf.LargestTextPaint(), start);
+        ClampDelta(perf.LargestTextPaint(), start);
     timing->paint_timing->largest_text_paint_size = perf.LargestTextPaintSize();
+    // LargestTextPaint and LargestTextPaintSize should be available at the
+    // same time. This is a renderer side DCHECK to ensure this.
+    DCHECK(perf.LargestTextPaintSize());
   }
   if (perf.ParseStart() > 0.0)
     timing->parse_timing->parse_start = ClampDelta(perf.ParseStart(), start);