Make base::Optional constructor constexpr.

base/optional.h

Index: base/optional.h
diff --git a/base/optional.h b/base/optional.h
index 3adda97c0217ee5c05d8c07a0fbaab062714d03a..8d172bbd9ff000b97494a2205865de08c293b1ff 100644
--- a/base/optional.h
+++ b/base/optional.h
@@ -34,7 +34,7 @@ namespace internal {
 
 template <typename T, bool = base::is_trivially_destructible<T>::value>
 struct OptionalStorage {
-  OptionalStorage() {}
+  constexpr OptionalStorage() {}
   // When T is not trivially destructible we must call its
   // destructor before deallocating its memory.
   ~OptionalStorage() {
@@ -54,7 +54,7 @@ struct OptionalStorage {
 
 template <typename T>
 struct OptionalStorage<T, true> {
-  OptionalStorage() {}
+  constexpr OptionalStorage() {}
   // When T is trivially destructible (i.e. its destructor does nothing) there
   // is no need to call it. Explicitly defaulting the destructor means it's not
   // user-provided. Those two together make this destructor trivial.