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/time_delta.h

@@ -32,7 +32,26 @@ namespace webrtc {
 // microseconds (us).
 class TimeDelta final : public rtc_units_impl::RelativeUnit<TimeDelta> {
  public:
+  template <typename T>
+  static constexpr TimeDelta Seconds(T value) {
+    static_assert(std::is_arithmetic<T>::value, "");
+    return FromFraction(1'000'000, value);
+  }
+  template <typename T>
+  static constexpr TimeDelta Millis(T value) {
+    static_assert(std::is_arithmetic<T>::value, "");
+    return FromFraction(1'000, value);
+  }
+  template <typename T>
+  static constexpr TimeDelta Micros(T value) {
+    static_assert(std::is_arithmetic<T>::value, "");
+    return FromValue(value);
+  }
+
   TimeDelta() = delete;
+
+  // TODO(danilchap): Migrate all code to the 3 factories above and delete the
+  // 6 factories below.
   template <int64_t seconds>
   static constexpr TimeDelta Seconds() {
     return FromFraction(1'000'000, seconds);