Add TimeDelta and Timestamp factories

These factories suppose to replace set of old constexpr factories that
takes parameter as template rather than function parameter,
as well as fix function naming to follow style guide of the second set
of factory functions.

Bug: None
Change-Id: Icd76302b821b2a4027f9d6765cf91bc9190f551c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/167521
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30482}

api/units/timestamp.h

@@ -28,8 +28,26 @@ namespace webrtc {
 // difference of two Timestamps results in a TimeDelta.
 class Timestamp final : public rtc_units_impl::UnitBase<Timestamp> {
  public:
+  template <typename T>
+  static constexpr Timestamp Seconds(T value) {
+    static_assert(std::is_arithmetic<T>::value, "");
+    return FromFraction(1'000'000, value);
+  }
+  template <typename T>
+  static constexpr Timestamp Millis(T value) {
+    static_assert(std::is_arithmetic<T>::value, "");
+    return FromFraction(1'000, value);
+  }
+  template <typename T>
+  static constexpr Timestamp Micros(T value) {
+    static_assert(std::is_arithmetic<T>::value, "");
+    return FromValue(value);
+  }
+
   Timestamp() = delete;
 
+  // TODO(danilchap): Migrate all code to the 3 factories above and delete the
+  // 6 factories below.
   template <int64_t seconds>
   static constexpr Timestamp Seconds() {
     return FromFraction(1'000'000, seconds);