Use the tonic namespace for tonic + resulting build fixes

BUG=
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1242773003 .
diff --git a/tonic/dart_class_library.cc b/tonic/dart_class_library.cc
index 60bc519..5dc1d29 100644
--- a/tonic/dart_class_library.cc
+++ b/tonic/dart_class_library.cc
@@ -7,7 +7,7 @@
 #include "base/logging.h"
 #include "tonic/dart_wrapper_info.h"
 
-namespace blink {
+namespace tonic {
 
 DartClassLibrary::DartClassLibrary() : provider_(nullptr) {
 }
@@ -31,4 +31,4 @@
   return result.first->second;
 }
 
-}  // namespace blink
+}  // namespace tonic