Use the tonic namespace for tonic + resulting build fixes

tonic/dart_gc_controller.cc

Index: tonic/dart_gc_controller.cc
diff --git a/tonic/dart_gc_controller.cc b/tonic/dart_gc_controller.cc
index 4b3505924641b1e6baa4f7271f9acfb7e7417556..d6d29fde7253d54b458d90434c6ace2870fd6608 100644
--- a/tonic/dart_gc_controller.cc
+++ b/tonic/dart_gc_controller.cc
@@ -10,7 +10,7 @@
 #include "tonic/dart_gc_visitor.h"
 #include "tonic/dart_wrappable.h"
 
-namespace blink {
+namespace tonic {
 namespace {
 
 DartGCContext* g_gc_context = nullptr;
@@ -49,4 +49,4 @@ void DartGCEpilogue() {
   TRACE_EVENT_ASYNC_END0("sky", "DartGC", 0);
 }
 
-}  // namespace blink
+}  // namespace tonic