Mark checked_cast, dchecked_cast, and saturated_cast as constexpr

to allow use them in other constexpr functions.
c++14 extends what can be in constexpr function making this change possible.

Bug: None
Change-Id: I6ae55b0b9b936021b57aa83ea5dd77d73be511a3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/159026
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29743}
diff --git a/rtc_base/numerics/safe_conversions.h b/rtc_base/numerics/safe_conversions.h
index 58efcaa..5d58672 100644
--- a/rtc_base/numerics/safe_conversions.h
+++ b/rtc_base/numerics/safe_conversions.h
@@ -23,7 +23,7 @@
 // Convenience function that returns true if the supplied value is in range
 // for the destination type.
 template <typename Dst, typename Src>
-inline bool IsValueInRangeForNumericType(Src value) {
+inline constexpr bool IsValueInRangeForNumericType(Src value) {
   return internal::RangeCheck<Dst>(value) == internal::TYPE_VALID;
 }
 
@@ -32,12 +32,12 @@
 // conversion will not overflow or underflow. NaN source will always trigger
 // the [D]CHECK.
 template <typename Dst, typename Src>
-inline Dst checked_cast(Src value) {
+inline constexpr Dst checked_cast(Src value) {
   RTC_CHECK(IsValueInRangeForNumericType<Dst>(value));
   return static_cast<Dst>(value);
 }
 template <typename Dst, typename Src>
-inline Dst dchecked_cast(Src value) {
+inline constexpr Dst dchecked_cast(Src value) {
   RTC_DCHECK(IsValueInRangeForNumericType<Dst>(value));
   return static_cast<Dst>(value);
 }
@@ -46,7 +46,7 @@
 // that the specified numeric conversion will saturate rather than overflow or
 // underflow. NaN assignment to an integral will trigger a RTC_CHECK condition.
 template <typename Dst, typename Src>
-inline Dst saturated_cast(Src value) {
+inline constexpr Dst saturated_cast(Src value) {
   // Optimization for floating point values, which already saturate.
   if (std::numeric_limits<Dst>::is_iec559)
     return static_cast<Dst>(value);