Fix base::Optional constexpr ctor on gcc 4.8.

base/optional.h

Index: base/optional.h
diff --git a/base/optional.h b/base/optional.h
index a32e06808d82cd7b75b66009880d891e4d62fceb..528610aa8e7a39c3f8808f9615cc1c6746d2f461 100644
--- a/base/optional.h
+++ b/base/optional.h
@@ -34,7 +34,8 @@ namespace internal {
 
 template <typename T, bool = base::is_trivially_destructible<T>::value>
 struct OptionalStorage {
-  constexpr OptionalStorage() {}
+  // Initializing |empty_| here to avoid errors in g++ 4.8.
+  constexpr OptionalStorage() : empty_('\0') {}
 
   constexpr explicit OptionalStorage(const T& value)
       : is_null_(false), value_(value) {}
@@ -67,7 +68,8 @@ struct OptionalStorage {
 
 template <typename T>
 struct OptionalStorage<T, true> {
-  constexpr OptionalStorage() {}
+  // Initializing |empty_| here to avoid errors in g++ 4.8.
+  constexpr OptionalStorage() : empty_('\0') {}
 
   constexpr explicit OptionalStorage(const T& value)
       : is_null_(false), value_(value) {}