Add support for tracking loading behavior of child frames.

chrome/renderer/page_load_metrics/page_timing_metrics_sender.cc

Index: chrome/renderer/page_load_metrics/page_timing_metrics_sender.cc
diff --git a/chrome/renderer/page_load_metrics/page_timing_metrics_sender.cc b/chrome/renderer/page_load_metrics/page_timing_metrics_sender.cc
index 00187883ce76f4d7d8d33c4a1e10c8f5d5629f61..e016bb3251445b98216fb664624a3b9a22bd3572 100644
--- a/chrome/renderer/page_load_metrics/page_timing_metrics_sender.cc
+++ b/chrome/renderer/page_load_metrics/page_timing_metrics_sender.cc
@@ -29,8 +29,10 @@ PageTimingMetricsSender::PageTimingMetricsSender(
       timer_(std::move(timer)),
       last_timing_(initial_timing),
       metadata_(PageLoadMetadata()) {
-  // Send an initial IPC relatively early to help track aborts.
-  EnsureSendTimer(kInitialTimerDelayMillis);
+  if (!initial_timing.IsEmpty()) {
+    // Send an initial IPC relatively early to help track aborts.
+    EnsureSendTimer(kInitialTimerDelayMillis);
+  }
 }
 
 // On destruction, we want to send any data we have if we have a timer