Mojo: Log an error when we fail to find a JS module.

gin/modules/file_module_provider.h

Index: gin/modules/file_module_provider.h
diff --git a/gin/modules/file_module_provider.h b/gin/modules/file_module_provider.h
index dd75a0feaf1ef2d7e0031533c15ae3e7cd0928a1..f3110dbe6cdb9c5e3905594d1cf7dea9b52c90e1 100644
--- a/gin/modules/file_module_provider.h
+++ b/gin/modules/file_module_provider.h
@@ -9,7 +9,9 @@
 #include <string>
 #include <vector>
 
+#include "base/callback.h"
 #include "base/files/file_path.h"
+#include "base/memory/weak_ptr.h"
 #include "gin/gin_export.h"
 #include "gin/runner.h"
 
@@ -27,15 +29,33 @@ class GIN_EXPORT FileModuleProvider {
 
   // Searches for modules with |ids| in the file system. If found, the modules
   // will be executed asynchronously by |runner|.
-  void AttempToLoadModules(Runner* runner, const std::set<std::string>& ids);
+  void AttemptToLoadModules(Runner* runner, const std::set<std::string>& ids);
+
+  void set_completion_callback(
+      base::Callback<void(const std::set<std::string>&)> callback) {
+    completion_callback_ = callback;
+  }
+  void reset_completion_callback() {
+    completion_callback_.Reset();
+  }
 
  private:
+  void OnReadCompleted(const std::string& id, bool success);
+
+  base::WeakPtrFactory<FileModuleProvider> weak_factory_;
+
   std::vector<base::FilePath> search_paths_;
 
   // We'll only search for a given module once. We remember the set of modules
   // we've already looked for in |attempted_ids_|.
   std::set<std::string> attempted_ids_;
 
+  // The set of modules that have succeeded or failed to load.
+  std::set<std::string> successful_ids_;
+  std::set<std::string> failed_ids_;
+
+  base::Callback<void(const std::set<std::string>&)> completion_callback_;
+
   DISALLOW_COPY_AND_ASSIGN(FileModuleProvider);
 };