Use the tonic namespace for tonic + resulting build fixes

tonic/dart_gc_context.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 <utility>
 
 #include "tonic/dart_gc_context.h"
 
-namespace blink {
+namespace tonic {
 
 DartGCContext::DartGCContext() : builder_(Dart_NewWeakReferenceSetBuilder()) {
 }
 
 DartGCContext::~DartGCContext() {
 }
 
 Dart_WeakReferenceSet DartGCContext::AddToSetForRoot(
     const void* root,
     Dart_WeakPersistentHandle handle) {
   const auto& result = references_.insert(std::make_pair(root, nullptr));
   if (!result.second) {
     // Already present.
     Dart_AppendToWeakReferenceSet(result.first->second, handle, handle);
     return result.first->second;
   }
   result.first->second = Dart_NewWeakReferenceSet(builder_, handle, handle);
   return result.first->second;
 }
 
-}  // namespace blink
+}  // namespace tonic