Address review comments on https://codereview.chromium.org/1077893005/

Add a comment to UpdateProcThreadAttribute() to note that its parameter needs
to outlive this.

BUG=476316

Review URL: https://codereview.chromium.org/1106263002

Cr-Commit-Position: refs/heads/master@{#327176}

base/win/startup_information.h

@@ -24,6 +24,8 @@ class BASE_EXPORT StartupInformation {
   bool InitializeProcThreadAttributeList(DWORD attribute_count);
 
   // Sets one entry in the initialized attribute list.
+  // |value| needs to live at least as long as the StartupInformation object
+  // this is called on.
   bool UpdateProcThreadAttribute(DWORD_PTR attribute,
                                  void* value,
                                  size_t size);