base: Make ElapsedTimer movable

Previously if you wanted to reset the starting point of the interval
(for measuring an idle timeout, for example) you had to construct a new
instance on the heap.

Now it's possible to keep a stack/member variable (potentially
base::Optional) and assign fresh values to it to reset it.

Review-Url: https://codereview.chromium.org/2851493004
Cr-Commit-Position: refs/heads/master@{#468193}

base/timer/elapsed_timer.cc

@@ -10,6 +10,14 @@ ElapsedTimer::ElapsedTimer() {
   begin_ = TimeTicks::Now();
 }
 
+ElapsedTimer::ElapsedTimer(ElapsedTimer&& other) {
+  begin_ = other.begin_;
+}
+
+void ElapsedTimer::operator=(ElapsedTimer&& other) {
+  begin_ = other.begin_;
+}
+
 TimeDelta ElapsedTimer::Elapsed() const {
   return TimeTicks::Now() - begin_;
 }

base/timer/elapsed_timer.h

@@ -15,6 +15,9 @@ namespace base {
 class BASE_EXPORT ElapsedTimer {
  public:
   ElapsedTimer();
+  ElapsedTimer(ElapsedTimer&& other);
+
+  void operator=(ElapsedTimer&& other);
 
   // Returns the time elapsed since object construction.
   TimeDelta Elapsed() const;