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/process_message_impl.h

@@ -7,20 +7,33 @@
 #pragma once
 
 #include "include/cef_process_message.h"
-#include "libcef/common/value_base.h"
 
-struct Cef_Request_Params;
+namespace base {
+class ListValue;
+}
 
-// CefProcessMessage implementation
-class CefProcessMessageImpl
-    : public CefValueBase<CefProcessMessage, Cef_Request_Params> {
+// CefProcessMessage implementation.
+class CefProcessMessageImpl : public CefProcessMessage {
  public:
-  CefProcessMessageImpl(Cef_Request_Params* value,
-                        bool will_delete,
+  // Constructor for an owned list of arguments.
+  CefProcessMessageImpl(const CefString& name,
+                        CefRefPtr<CefListValue> arguments);
+
+  // Constructor for an unowned list of arguments.
+  // Call Detach() when |arguments| is no longer valid.
+  CefProcessMessageImpl(const CefString& name,
+                        base::ListValue* arguments,
                         bool read_only);
 
-  // Copies the underlying value to the specified |target| structure.
-  bool CopyTo(Cef_Request_Params& target);
+  ~CefProcessMessageImpl() override;
+
+  // Stop referencing the underlying argument list (which we never owned).
+  void Detach();
+
+  // Transfer ownership of the underlying argument list to the caller.
+  // TODO: Pass by reference instead of ownership if/when Mojo adds support
+  // for that.
+  base::ListValue TakeArgumentList() WARN_UNUSED_RESULT;
 
   // CefProcessMessage methods.
   bool IsValid() override;
@@ -29,6 +42,12 @@ class CefProcessMessageImpl
   CefString GetName() override;
   CefRefPtr<CefListValue> GetArgumentList() override;
 
+ private:
+  const CefString name_;
+  CefRefPtr<CefListValue> arguments_;
+  const bool should_detach_ = false;
+
+  IMPLEMENT_REFCOUNTING(CefProcessMessageImpl);
   DISALLOW_COPY_AND_ASSIGN(CefProcessMessageImpl);
 };