Make base::internal::CallbackBase::is_null inline.

This saves 11 bytes per call site on x64-clang and 12 byte per call site on
ARM-gcc.

BUG=410776
TEST=base_unittests

Review URL: https://codereview.chromium.org/540883002

Cr-Commit-Position: refs/heads/master@{#295215}


CrOS-Libchrome-Original-Commit: caf874447fabaaee90921d611a052648ea150c70
diff --git a/base/callback_internal.h b/base/callback_internal.h
index e66623c..9d7761c 100644
--- a/base/callback_internal.h
+++ b/base/callback_internal.h
@@ -37,7 +37,7 @@
 class BASE_EXPORT CallbackBase {
  public:
   // Returns true if Callback is null (doesn't refer to anything).
-  bool is_null() const;
+  bool is_null() const { return bind_state_.get() == NULL; }
 
   // Returns the Callback into an uninitialized state.
   void Reset();