Use a TickClock instead of TimeTicks::Now() in Timer.

base/timer/timer.h

Index: base/timer/timer.h
diff --git a/base/timer/timer.h b/base/timer/timer.h
index 661829b513d8281f99bb63741b91f9c026e6ff39..20cce8c69df37d02eb8e901ecf06662b0d5dae6e 100644
--- a/base/timer/timer.h
+++ b/base/timer/timer.h
@@ -49,6 +49,8 @@
 // because they're flaky on the buildbot, but when you run them locally you
 // should be able to tell the difference.
 
+#include <memory>
+
 #include "base/base_export.h"
 #include "base/bind.h"
 #include "base/bind_helpers.h"
@@ -61,6 +63,7 @@ namespace base {
 
 class BaseTimerTaskInternal;
 class SingleThreadTaskRunner;
+class TickClock;
 
 //-----------------------------------------------------------------------------
 // This class wraps MessageLoop::PostDelayedTask to manage delayed and repeating
@@ -93,6 +96,9 @@ class BASE_EXPORT Timer {
   // run tasks on the same thread the timer is used on.
   virtual void SetTaskRunner(scoped_refptr<SingleThreadTaskRunner> task_runner);
 
+  // Sets the tick clock used to calculate the run time for scheduled tasks.
+  virtual void SetTickClock(std::unique_ptr<TickClock> tick_clock);

[danakj, 2016/11/09 19:57:38]

This class being virtual means subclasses need to use the tick clock too.

Here's one example I found through codesearch:
https://cs.chromium.org/chromium/src/components/timers/alarm_timer_chromeos.c...

How can we help ensure subclasses do the right thing?

Here's one idea:
1. Add a protected pure-virtual-but-implemented method Now() that calls
TimeTicks::Now(). (looks like "virtual base::TimeTicks Now() = 0;" and a defn of
that in the .cc file)
2. Subclasses will not compile without making a definition of Now() themselves.
Production can call the base class (and convert direct calls to TimeTicks::Now()
to "Now()" in the process. Tests can do whatever they like.

What do you think?

[jameswest, 2016/11/10 00:26:05]

I'm fine with that idea, but it's currently possible to instantiate Timer
directly because it doesn't have any pure virtual methods. If we add the pure
virtual method, that will no longer be possible. I don't think anything directly
instantiates Timer other than its unit tests, but are we OK with requiring only
subclasses be used?

+
   // Start the timer to run at the given |delay| from now. If the timer is
   // already running, it will be replaced to call the given |user_task|.
   virtual void Start(const tracked_objects::Location& posted_from,
@@ -160,6 +166,9 @@ class BASE_EXPORT Timer {
   // task runner for the current thread should be used.
   scoped_refptr<SingleThreadTaskRunner> task_runner_;
 
+  // The tick clock used to calculate the run time for scheduled tasks.
+  std::unique_ptr<TickClock> tick_clock_;
+
   // Location in user code.
   tracked_objects::Location posted_from_;
   // Delay requested by user.