Add support for writing unit tests for Mojo-backed apps/extensions APIs.

extensions/test/extensions_unittests_main.cc

Index: extensions/test/extensions_unittests_main.cc
diff --git a/extensions/test/extensions_unittests_main.cc b/extensions/test/extensions_unittests_main.cc
index bc7afc56d7a585da41234f26962e8e3dc621a103..af252571540083796bd6c83f3c494dd59fa02bb6 100644
--- a/extensions/test/extensions_unittests_main.cc
+++ b/extensions/test/extensions_unittests_main.cc
@@ -11,6 +11,7 @@
 #include "content/public/test/unittest_test_suite.h"
 #include "extensions/common/extension_paths.h"
 #include "extensions/test/test_extensions_client.h"
+#include "mojo/embedder/embedder.h"
 #include "ui/base/resource/resource_bundle.h"
 
 namespace {
@@ -56,6 +57,7 @@ void ExtensionsTestSuite::Shutdown() {
 int main(int argc, char** argv) {
   content::UnitTestTestSuite test_suite(new ExtensionsTestSuite(argc, argv));
 
+  mojo::embedder::Init();
   return base::LaunchUnitTests(argc,
                                argv,
                                base::Bind(&content::UnitTestTestSuite::Run,