Adds constexpr create functions for units.

This adds new constexpr create function for DataSize, DataRate,
TimeDelta and Timestamp. The names are capitalized to mirror the
naming scheme of the previously constexpr methods (Zero and
Infinity create functions). They are also kept longer since they
are not expected to be used in complex expressions.

Bug: webrtc:9574
Change-Id: I5950548718675050fc5d66699de295455c310861
Reviewed-on: https://webrtc-review.googlesource.com/91161
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24218}
diff --git a/api/units/timestamp_unittest.cc b/api/units/timestamp_unittest.cc
index 7dfe669..db894cc 100644
--- a/api/units/timestamp_unittest.cc
+++ b/api/units/timestamp_unittest.cc
@@ -14,8 +14,24 @@
 namespace webrtc {
 namespace test {
 TEST(TimestampTest, ConstExpr) {
+  constexpr int64_t kValue = 12345;
   constexpr Timestamp kTimestampInf = Timestamp::Infinity();
   static_assert(kTimestampInf.IsInfinite(), "");
+  static_assert(kTimestampInf.ms_or(-1) == -1, "");
+
+  constexpr Timestamp kTimestampSeconds = Timestamp::Seconds<kValue>();
+  constexpr Timestamp kTimestampMs = Timestamp::Millis<kValue>();
+  constexpr Timestamp kTimestampUs = Timestamp::Micros<kValue>();
+
+  static_assert(kTimestampSeconds.seconds_or(0) == kValue, "");
+  static_assert(kTimestampMs.ms_or(0) == kValue, "");
+  static_assert(kTimestampUs.us_or(0) == kValue, "");
+
+  static_assert(kTimestampMs > kTimestampUs, "");
+
+  EXPECT_EQ(kTimestampSeconds.seconds(), kValue);
+  EXPECT_EQ(kTimestampMs.ms(), kValue);
+  EXPECT_EQ(kTimestampUs.us(), kValue);
 }
 
 TEST(TimestampTest, GetBackSameValues) {