Base: Mark time constants as constexpr.

This will allow use of these constants in constexpr expressions.

Change-Id: Ia2d6f8ab6969eedbc9b9b763f7f700ffaaaee14b
Reviewed-on: https://chromium-review.googlesource.com/1219769Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590419}

base/time/time.h

@@ -320,20 +320,20 @@ namespace time_internal {
 template<class TimeClass>
 class TimeBase {
  public:
-  static const int64_t kHoursPerDay = 24;
-  static const int64_t kMillisecondsPerSecond = 1000;
-  static const int64_t kMillisecondsPerDay =
+  static constexpr int64_t kHoursPerDay = 24;
+  static constexpr int64_t kMillisecondsPerSecond = 1000;
+  static constexpr int64_t kMillisecondsPerDay =
       kMillisecondsPerSecond * 60 * 60 * kHoursPerDay;
-  static const int64_t kMicrosecondsPerMillisecond = 1000;
-  static const int64_t kMicrosecondsPerSecond =
+  static constexpr int64_t kMicrosecondsPerMillisecond = 1000;
+  static constexpr int64_t kMicrosecondsPerSecond =
       kMicrosecondsPerMillisecond * kMillisecondsPerSecond;
-  static const int64_t kMicrosecondsPerMinute = kMicrosecondsPerSecond * 60;
-  static const int64_t kMicrosecondsPerHour = kMicrosecondsPerMinute * 60;
-  static const int64_t kMicrosecondsPerDay =
+  static constexpr int64_t kMicrosecondsPerMinute = kMicrosecondsPerSecond * 60;
+  static constexpr int64_t kMicrosecondsPerHour = kMicrosecondsPerMinute * 60;
+  static constexpr int64_t kMicrosecondsPerDay =
       kMicrosecondsPerHour * kHoursPerDay;
-  static const int64_t kMicrosecondsPerWeek = kMicrosecondsPerDay * 7;
-  static const int64_t kNanosecondsPerMicrosecond = 1000;
-  static const int64_t kNanosecondsPerSecond =
+  static constexpr int64_t kMicrosecondsPerWeek = kMicrosecondsPerDay * 7;
+  static constexpr int64_t kNanosecondsPerMicrosecond = 1000;
+  static constexpr int64_t kNanosecondsPerSecond =
       kNanosecondsPerMicrosecond * kMicrosecondsPerSecond;
 
   // Returns true if this object has not been initialized.