base: Suggest forward declare DefaultSingletonTraits in header files.

Instead of including singleton.h in header files we can forward declare it,
and include only in the source file.

There is also a presubmit check for this, I didn't know ;)

R=willchan@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10835029

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@149095 0039d316-1c4b-4281-b951-d872f2087c98

base/memory/singleton.h

@@ -149,7 +149,7 @@ template <typename Type> base::subtle::Atomic32
 // Example usage:
 //
 // In your header:
-//   #include "base/memory/singleton.h"
+//   template <typename T> struct DefaultSingletonTraits;
 //   class FooClass {
 //    public:
 //     static FooClass* GetInstance();  <-- See comment below on this.
@@ -162,6 +162,7 @@ template <typename Type> base::subtle::Atomic32
 //   };
 //
 // In your source file:
+//  #include "base/memory/singleton.h"
 //  FooClass* FooClass::GetInstance() {
 //    return Singleton<FooClass>::get();
 //  }