GetTimeToCallback() returns estimated time to callback.

base/timer/timer.cc

Index: base/timer/timer.cc
diff --git a/base/timer/timer.cc b/base/timer/timer.cc
index 6ec18f181488c3a5b533abe93cf9b793914e6451..6e86c11c5d06ae15312a5108757888ac04675bbd 100644
--- a/base/timer/timer.cc
+++ b/base/timer/timer.cc
@@ -107,6 +107,12 @@ TimeDelta Timer::GetCurrentDelay() const {
   return delay_;
 }
 
+TimeDelta Timer::GetTimeToCallback() const {
+  if (!user_task_)
+    return TimeDelta::Max();
+  return scheduled_run_time_ - Now();
+}
+
 void Timer::SetTaskRunner(scoped_refptr<SingleThreadTaskRunner> task_runner) {
   // Do not allow changing the task runner once something has been scheduled.
   DCHECK_EQ(thread_id_, 0);