Convert base::Clock::Now() a const method

This method should be a const method to be consistent to NowTicks() in
base::TickClock, which is also converted to a const method at
http://crrev.com/821f0e338ebbe420

Bug: 789079
Change-Id: I6b5c9bcee2d730214298ae7a7136ea6b312ad079
Reviewed-on: https://chromium-review.googlesource.com/1001213
Commit-Queue: Taiju Tsuiki <[email protected]>
Reviewed-by: Helen Li <[email protected]>
Reviewed-by: Gabriel Charette <[email protected]>
Reviewed-by: Jochen Eisinger <[email protected]>
Cr-Commit-Position: refs/heads/master@{#550598}
diff --git a/components/gcm_driver/gcm_client_impl_unittest.cc b/components/gcm_driver/gcm_client_impl_unittest.cc
index 85a8b6f..b2f77a88 100644
--- a/components/gcm_driver/gcm_client_impl_unittest.cc
+++ b/components/gcm_driver/gcm_client_impl_unittest.cc
@@ -181,14 +181,14 @@
   explicit AutoAdvancingTestClock(base::TimeDelta auto_increment_time_delta);
   ~AutoAdvancingTestClock() override;
 
-  base::Time Now() override;
+  base::Time Now() const override;
   void Advance(TimeDelta delta);
   int call_count() const { return call_count_; }
 
  private:
-  int call_count_;
+  mutable int call_count_;
   base::TimeDelta auto_increment_time_delta_;
-  base::Time now_;
+  mutable base::Time now_;
 
   DISALLOW_COPY_AND_ASSIGN(AutoAdvancingTestClock);
 };
@@ -201,7 +201,7 @@
 AutoAdvancingTestClock::~AutoAdvancingTestClock() {
 }
 
-base::Time AutoAdvancingTestClock::Now() {
+base::Time AutoAdvancingTestClock::Now() const {
   call_count_++;
   now_ += auto_increment_time_delta_;
   return now_;