Fix newly discovered presubmit errors.

src/base/platform/platform.h

Index: src/base/platform/platform.h
diff --git a/src/base/platform/platform.h b/src/base/platform/platform.h
index 1645a4de08d761d9fb7a23710eebb77b6a74e1ff..8a5412626857af462184c0d916819ddfe4c3b9dd 100644
--- a/src/base/platform/platform.h
+++ b/src/base/platform/platform.h
@@ -428,7 +428,7 @@ class Thread {
   class Options {
    public:
     Options() : name_("v8:<unknown>"), stack_size_(0) {}
-    Options(const char* name, int stack_size = 0)
+    explicit Options(const char* name, int stack_size = 0)
         : name_(name), stack_size_(stack_size) {}
 
     const char* name() const { return name_; }