Convert legacy IPC messages to Mojo (fixes issue #3123)

This change introduces a few minor CEF API behavior changes:

- A CefProcessMessage object cannot be reused after being passed to
  SendProcessMessage.
- The |extra_info| argument to CefRenderProcessHandler::OnBrowserCreated may
  now be NULL.

Where appropriate, we now utilize the default UTF string encoding format and
shared memory to reduce copies and conversions for the cross-process
transfer of arbitrary-length strings. For example, CefFrame::GetSource/GetText
now involves zero UTF conversions and zero copies in the browser process for
the CefString delivered to CefStringVisitor::Visit().

libcef/common/string_util.h

@@ -0,0 +1,44 @@
+// Copyright 2021 The Chromium Embedded Framework Authors. Portions copyright
+// 2011 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 CEF_LIBCEF_COMMON_STRING_UTIL_H_
+#define CEF_LIBCEF_COMMON_STRING_UTIL_H_
+#pragma once
+
+#include "include/cef_base.h"
+
+#include "base/callback.h"
+#include "base/memory/scoped_refptr.h"
+
+namespace base {
+class ReadOnlySharedMemoryRegion;
+class RefCountedMemory;
+}  // namespace base
+
+namespace blink {
+class WebString;
+}
+
+namespace string_util {
+
+// Convert |source| to |cef_string|, avoiding UTF conversions if possible.
+void GetCefString(const blink::WebString& source, CefString& cef_string);
+void GetCefString(scoped_refptr<base::RefCountedMemory> source,
+                  CefString& cef_string);
+
+// Read |source| into shared memory, avoiding UTF conversions if possible.
+// Use ExecuteWithScopedCefString() to retrieve the value on the receiving end
+// with zero UTF conversions and zero copies if possible.
+base::ReadOnlySharedMemoryRegion CreateSharedMemoryRegion(
+    const blink::WebString& source);
+
+using ScopedCefStringCallback = base::OnceCallback<void(const CefString&)>;
+
+// Helper for executing |callback| with |region| as a scoped CefString.
+void ExecuteWithScopedCefString(base::ReadOnlySharedMemoryRegion region,
+                                ScopedCefStringCallback callback);
+
+}  // namespace string_util
+
+#endif  // CEF_LIBCEF_COMMON_STRING_UTIL_H_