Remove some UI->FILE->UI thread hops in ExecuteCodeFunction

extensions/browser/file_reader.cc

Index: extensions/browser/file_reader.cc
diff --git a/extensions/browser/file_reader.cc b/extensions/browser/file_reader.cc
index c5deb9b646e071cc844cceb9ec57922b35afab31..3efc7d51a27e7c8728e23cc46d7cbe8816728013 100644
--- a/extensions/browser/file_reader.cc
+++ b/extensions/browser/file_reader.cc
@@ -5,16 +5,20 @@
 #include "extensions/browser/file_reader.h"
 
 #include "base/bind.h"
+#include "base/callback_helpers.h"
 #include "base/files/file_util.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "content/public/browser/browser_thread.h"
 
 using content::BrowserThread;
 
-FileReader::FileReader(const extensions::ExtensionResource& resource,
-                       const Callback& callback)
+FileReader::FileReader(
+    const extensions::ExtensionResource& resource,
+    const OptionalFileThreadTaskCallback& optional_file_thread_task_callback,
+    const DoneCallback& done_callback)
     : resource_(resource),
-      callback_(callback),
+      optional_file_thread_task_callback_(optional_file_thread_task_callback),
+      done_callback_(done_callback),
       origin_task_runner_(base::ThreadTaskRunnerHandle::Get()) {}
 
 void FileReader::Start() {
@@ -28,6 +32,17 @@ FileReader::~FileReader() {}
 void FileReader::ReadFileOnBackgroundThread() {
   std::unique_ptr<std::string> data(new std::string());
   bool success = base::ReadFileToString(resource_.GetFilePath(), data.get());
+
+  if (!optional_file_thread_task_callback_.is_null()) {
+    if (success) {
+      base::ResetAndReturn(&optional_file_thread_task_callback_)
+          .Run(data.get());
+    } else {
+      optional_file_thread_task_callback_.Reset();
+    }
+  }
+
   origin_task_runner_->PostTask(
-      FROM_HERE, base::Bind(callback_, success, base::Passed(std::move(data))));
+      FROM_HERE, base::Bind(base::ResetAndReturn(&done_callback_), success,
+                            base::Passed(std::move(data))));
 }