Mark remaining mojo_base time primitives as Stable.

Before the change, the "Time" struct from
//mojo/public/mojom/base/time.mojom is marked as a Stable interface,
while the other two testructs (TimeDelta and TimeTicks) are not.

This adds consistency  and allows stable interfaces to depend
on these basic types.

Change-Id: Icae41a542bddde9e6e8ab7b63cb01d4af3a4662f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517479
Commit-Queue: Krzysztof Olczyk <kolczyk@vewd.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823924}

mojo/public/mojom/base/time.mojom

@@ -11,10 +11,12 @@ struct Time {
   int64 internal_value;
 };
 
+[Stable]
 struct TimeDelta {
   int64 microseconds;
 };
 
+[Stable]
 struct TimeTicks {
   // The internal value is expressed in terms of microseconds since a fixed but
   // intentionally unspecified epoch.