Bind ModelLoader to a sequence, not a thread.

chrome/browser/safe_browsing/client_side_model_loader.h

Index: chrome/browser/safe_browsing/client_side_model_loader.h
diff --git a/chrome/browser/safe_browsing/client_side_model_loader.h b/chrome/browser/safe_browsing/client_side_model_loader.h
index 3fc71836494c0354e62f6d82ea9e6363e91b148a..720232c303a0fb4bfb262fa0e180e9c73501535b 100644
--- a/chrome/browser/safe_browsing/client_side_model_loader.h
+++ b/chrome/browser/safe_browsing/client_side_model_loader.h
@@ -4,9 +4,6 @@
 //
 // Helper class loads models for client-side phishing detection
 // from the the SafeBrowsing backends.
-//
-// This class is not thread-safe and expects all calls to be made on the UI
-// thread.
 
 #ifndef CHROME_BROWSER_SAFE_BROWSING_CLIENT_SIDE_MODEL_LOADER_H_
 #define CHROME_BROWSER_SAFE_BROWSING_CLIENT_SIDE_MODEL_LOADER_H_
@@ -22,7 +19,7 @@
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
-#include "content/public/browser/browser_thread.h"
+#include "base/sequence_checker.h"
 #include "net/url_request/url_fetcher_delegate.h"
 #include "url/gurl.h"
 
@@ -49,6 +46,9 @@ class ModelLoader : public net::URLFetcherDelegate {
   static const char kClientModelUrlPrefix[];
   static const char kClientModelNamePattern[];
 
+  // Construct a model loader to fetch a model using |request_context_getter|.
+  // When ScheduleFetch is called, |update_renderers| will be called on the
+  // same sequence if the fetch is successful.
   ModelLoader(base::Closure update_renderers,
               net::URLRequestContextGetter* request_context_getter,
               bool is_extended_reporting);
@@ -60,7 +60,8 @@ class ModelLoader : public net::URLFetcherDelegate {
   // Schedules the next fetch of the model.
   virtual void ScheduleFetch(int64_t delay_ms);
 
-  // Cancel any pending model fetch.
+  // Cancel any pending model fetch. This must be called from the same sequence
+  // as ScheduleFetch.
   virtual void CancelFetcher();
 
   const std::string& model_str() const { return model_str_; }
@@ -95,6 +96,11 @@ class ModelLoader : public net::URLFetcherDelegate {
   virtual void EndFetch(ClientModelStatus status, base::TimeDelta max_age);
 
  private:
+  // Common constructor called from both the public and test-only constructors.
+  ModelLoader(base::Closure update_renderers,
+              net::URLRequestContextGetter* request_context_getter,
+              const std::string& model_name);
+
   // Use Finch to pick a model number.
   static int GetModelNumber();
 
@@ -125,6 +131,10 @@ class ModelLoader : public net::URLFetcherDelegate {
   // Used to protect the delayed callback to StartFetchModel()
   base::WeakPtrFactory<ModelLoader> weak_factory_;
 
+  // Used to check that ScheduleFetch and CancelFetcher are called on the same
+  // sequence.
+  base::SequenceChecker fetch_sequence_checker_;
+
   friend class ClientSideDetectionServiceTest;
   friend class ModelLoaderTest;
   FRIEND_TEST_ALL_PREFIXES(ModelLoaderTest, FetchModelTest);