Allow backoff entry serialization if clock is null.

net/base/backoff_entry.h

Index: net/base/backoff_entry.h
diff --git a/net/base/backoff_entry.h b/net/base/backoff_entry.h
index 3efd32cee01468fb470e180db645cea27f58102e..c15fa7a8659889d37c0a46b0afa9f3f12d19326b 100644
--- a/net/base/backoff_entry.h
+++ b/net/base/backoff_entry.h
@@ -24,6 +24,8 @@ namespace net {
 // This utility class knows nothing about network specifics; it is
 // intended for reuse in various networking scenarios.
 class NET_EXPORT BackoffEntry : NON_EXPORTED_BASE(public base::NonThreadSafe) {
+  friend class BackoffEntrySerializer;

[johnme, 2016/10/03 12:18:22]

Reviewers in https://codereview.chromium.org/1023473003#msg13 had a preference
for avoiding friend classes unless really needed. It might make sense to make
GetTimeTicksNow() public instead of friending? Alternatively, if you do make
BackoffEntrySerializer a friend class, you should probably make
BackoffDurationToReleaseTime private (it's only currently public so
BackoffEntrySerializer can use it).

[Olivier, 2016/10/03 12:29:24]

Done.

+
  public:
   // The set of parameters that define a back-off policy. When modifying this,
   // increment SERIALIZATION_VERSION_NUMBER in backoff_entry_serializer.cc.