[ios] Adds OmahaServiceProvider.

ios/public/provider/chrome/browser/omaha/omaha_xml_writer.h

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef IOS_PUBLIC_PROVIDER_CHROME_BROWSER_OMAHA_OMAHA_XML_WRITER_H_
+#define IOS_PUBLIC_PROVIDER_CHROME_BROWSER_OMAHA_OMAHA_XML_WRITER_H_
+
+// OmahaXmlWriter is a helper class to generate XML for the Omaha request.
+class OmahaXmlWriter {
+ public:
+  OmahaXmlWriter() = default;
+  virtual ~OmahaXmlWriter() = default;
+
+  // Starts a new element with the given |name|.
+  virtual void StartElement(const char* name) = 0;
+
+  // Ends the current element.
+  virtual void EndElement() = 0;
+
+  // Writes an attribute with the given |name| and |value| to the current
+  // element.
+  virtual void WriteAttribute(const char* name, const char* value) = 0;
+
+  // Finalizes the XML document.  Calling StartElement(), EndElement(), or
+  // WriteAttribute() will have no effect after calling this method.
+  virtual void Finalize() = 0;
+
+  // Returns the generated XML. This method should only be called after
+  // Finalize().
+  virtual std::string GetContentAsString() = 0;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(OmahaXmlWriter);
+};
+
+#endif  // IOS_PUBLIC_PROVIDER_CHROME_BROWSER_OMAHA_OMAHA_XML_WRITER_H_