Crash fix in lazy load PLM

There is a missing nullptr check in the render frame
observer. If the sender does not exist, we won't try
to call into it

Bug: 949507
Change-Id: I92af9c53af082800155b955e74ac0ac771d4369e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1553100
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Auto-Submit: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#647728}

chrome/renderer/page_load_metrics/metrics_render_frame_observer.cc

@@ -108,7 +108,8 @@ void MetricsRenderFrameObserver::DidObserveLayoutJank(double jank_fraction) {
 
 void MetricsRenderFrameObserver::DidObserveLazyLoadBehavior(
     blink::WebLocalFrameClient::LazyLoadBehavior lazy_load_behavior) {
-  page_timing_metrics_sender_->DidObserveLazyLoadBehavior(lazy_load_behavior);
+  if (page_timing_metrics_sender_)
+    page_timing_metrics_sender_->DidObserveLazyLoadBehavior(lazy_load_behavior);
 }
 
 void MetricsRenderFrameObserver::DidStartResponse(