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}

base/callback_internal.cc

@@ -9,10 +9,6 @@
 namespace base {
 namespace internal {
 
-bool CallbackBase::is_null() const {
-  return bind_state_.get() == NULL;
-}
-
 void CallbackBase::Reset() {
   polymorphic_invoke_ = NULL;
   // NULL the bind_state_ last, since it may be holding the last ref to whatever

base/callback_internal.h

@@ -37,7 +37,7 @@ class BindStateBase : public RefCountedThreadSafe<BindStateBase> {
 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();