Use the tonic namespace for tonic + resulting build fixes

tonic/dart_persistent_value.h

Index: tonic/dart_persistent_value.h
diff --git a/tonic/dart_persistent_value.h b/tonic/dart_persistent_value.h
index c6215c4b077a42f11febc19f94f0846e591118bd..759cb61541c642bb6ee01755905f434e0426aaa8 100644
--- a/tonic/dart_persistent_value.h
+++ b/tonic/dart_persistent_value.h
@@ -9,7 +9,7 @@
 #include "base/memory/weak_ptr.h"
 #include "dart/runtime/include/dart_api.h"
 
-namespace blink {
+namespace tonic {
 class DartState;
 
 // DartPersistentValue is a bookkeeping class to help pair calls to
@@ -38,6 +38,6 @@ class DartPersistentValue {
   DISALLOW_COPY_AND_ASSIGN(DartPersistentValue);
 };
 
-}  // namespace blink
+}  // namespace tonic
 
 #endif  // SKY_ENGINE_TONIC_DART_PERSISTENT_VALUE_H_