Remove explicit from Optional constructors

This allows base::nullopt to be used instead of base::Optional<Foo>()
and implicit conversion to Optional<T> from T.

Also added NOLINT comments to the implicit constructors.

Change-Id: I4c688045685e2a50e0c0d38a959570f77454ec61
Reviewed-on: https://chromium-review.googlesource.com/893268Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51003}

src/base/optional.h

@@ -125,7 +125,7 @@ class Optional {
 
   constexpr Optional() {}
 
-  explicit constexpr Optional(base::nullopt_t) {}
+  constexpr Optional(base::nullopt_t) {}  // NOLINT(runtime/explicit)
 
   Optional(const Optional& other) {
     if (!other.storage_.is_null_) Init(other.value());
@@ -135,10 +135,12 @@ class Optional {
     if (!other.storage_.is_null_) Init(std::move(other.value()));
   }
 
-  explicit constexpr Optional(const T& value) : storage_(value) {}
+  constexpr Optional(const T& value)  // NOLINT(runtime/explicit)
+      : storage_(value) {}
 
   // TODO(alshabalin): Can't use 'constexpr' with std::move until C++14.
-  explicit Optional(T&& value) : storage_(std::move(value)) {}
+  Optional(T&& value)  // NOLINT(runtime/explicit)
+      : storage_(std::move(value)) {}
 
   // TODO(alshabalin): Can't use 'constexpr' with std::forward until C++14.
   template <class... Args>