Allow backoff entry serialization if clock is null.

net/base/backoff_entry_serializer.cc

Index: net/base/backoff_entry_serializer.cc
diff --git a/net/base/backoff_entry_serializer.cc b/net/base/backoff_entry_serializer.cc
index acd6e9f9a7cf8513e7c10accf6f98edf97a1aad6..f2cbec753e092803651a2c5755a91120c508c5fd 100644
--- a/net/base/backoff_entry_serializer.cc
+++ b/net/base/backoff_entry_serializer.cc
@@ -29,7 +29,7 @@ std::unique_ptr<base::Value> BackoffEntrySerializer::SerializeToValue(
 
   // Can't use entry.GetTimeUntilRelease as it doesn't allow negative deltas.
   base::TimeDelta backoff_duration =
-      entry.GetReleaseTime() - entry.tick_clock()->NowTicks();
+      entry.GetReleaseTime() - entry.GetTimeTicksNow();
   // Redundantly stores both the remaining time delta and the absolute time.
   // The delta is used to work around some cases where wall clock time changes.
   serialized->AppendDouble(backoff_duration.InSecondsF());