Grab ThreadTaskRunnerHandle in JsonSanitizerAndroid before calling into Java.

This works around an issue where in official builds of unit_tests the
ThreadTaskRunnerHandle is null when calling into native code from Java.

Bug: 739510
Change-Id: Ide93e67076f905d03a6a9271a812e49553dcf0d8
Reviewed-on: https://chromium-review.googlesource.com/570251Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Bernhard Bauer <bauerb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486750}

components/safe_json/json_sanitizer_android.cc

@@ -51,6 +51,11 @@ class JsonSanitizerAndroid : public JsonSanitizer {
   StringCallback success_callback_;
   StringCallback error_callback_;
 
+  // Runs the callbacks. Stored as a member because grabbing it when posting
+  // the callbacks (being called from Java) fails under certain circumstances
+  // (https://crbug.com/739510).
+  scoped_refptr<base::SequencedTaskRunner> task_runner_;
+
   DISALLOW_COPY_AND_ASSIGN(JsonSanitizerAndroid);
 };
 
@@ -58,7 +63,8 @@ JsonSanitizerAndroid::JsonSanitizerAndroid(
     const StringCallback& success_callback,
     const StringCallback& error_callback)
     : success_callback_(success_callback),
-      error_callback_(error_callback) {}
+      error_callback_(error_callback),
+      task_runner_(base::SequencedTaskRunnerHandle::Get()) {}
 
 void JsonSanitizerAndroid::Sanitize(const std::string& unsafe_json) {
   // The JSON parser only accepts wellformed UTF-8.
@@ -77,13 +83,11 @@ void JsonSanitizerAndroid::Sanitize(const std::string& unsafe_json) {
 }
 
 void JsonSanitizerAndroid::OnSuccess(const std::string& json) {
-  base::SequencedTaskRunnerHandle::Get()->PostTask(
-      FROM_HERE, base::Bind(success_callback_, json));
+  task_runner_->PostTask(FROM_HERE, base::Bind(success_callback_, json));
 }
 
 void JsonSanitizerAndroid::OnError(const std::string& error) {
-  base::SequencedTaskRunnerHandle::Get()->PostTask(
-      FROM_HERE, base::Bind(error_callback_, error));
+  task_runner_->PostTask(FROM_HERE, base::Bind(error_callback_, error));
 }
 
 }  // namespace
@@ -111,7 +115,7 @@ void JsonSanitizer::Sanitize(const std::string& unsafe_json,
                              const StringCallback& success_callback,
                              const StringCallback& error_callback) {
   // JsonSanitizerAndroid does all its work synchronously, but posts any
-  // callbacks to the current message loop. This means it can be destroyed at
+  // callbacks to the current sequence. This means it can be destroyed at
   // the end of this method.
   JsonSanitizerAndroid sanitizer(success_callback, error_callback);
   sanitizer.Sanitize(unsafe_json);