Use the tonic namespace for tonic + resulting build fixes

tonic/dart_gc_visitor.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tonic/dart_gc_visitor.h"
 
 #include "tonic/dart_gc_context.h"
 
-namespace blink {
+namespace tonic {
 
 DartGCVisitor::DartGCVisitor(DartGCContext* context)
     : context_(context), current_set_(nullptr) {
 }
 
 DartGCVisitor::~DartGCVisitor() {
 }
 
 void DartGCVisitor::AddToSetForRoot(const void* root,
                                     Dart_WeakPersistentHandle handle) {
   Dart_WeakReferenceSet set = context_->AddToSetForRoot(root, handle);
   DCHECK(!current_set_ || current_set_ == set);
   current_set_ = set;
 }
 
-}  // namespace blink
+}  // namespace tonic