macOS: Add support for building clients with ARC enabled (fixes issue #2623).

Under ARC (Automatic Reference Counting), assigning to an Objective-C
pointer has different semantics than assigning to a void* pointer.
This makes it dangerous to treat the same memory address as an
Objective-C pointer in some cases and as a "regular C pointer" in
other cases.

This change removes the conditional type defines and instead uses
void* everywhere. Explicit type casting in combination with ARC
annotations makes it safe to get typed Objective-C pointers from the
void* pointers.

This change enables ARC by default in the CEF binary distribution CMake
configuration for the cefclient and cefsimple sample applications. It can be
disabled by adding `-DOPTION_USE_ARC=Off` to the CMake command line.

ARC is not supported when building Chromium due to the substantial
number of changes that would be required in the Chromium code base.

tests/cefclient/browser/browser_window_osr_mac.h

@@ -13,6 +13,8 @@
 
 namespace client {
 
+class BrowserWindowOsrMacImpl;
+
 // Represents a native child window hosting a single off-screen browser
 // instance. The methods of this class must be called on the main thread unless
 // otherwise indicated.
@@ -88,17 +90,11 @@ class BrowserWindowOsrMac : public BrowserWindow,
   void UpdateAccessibilityLocation(CefRefPtr<CefValue> value) OVERRIDE;
 
  private:
-  // Create the NSView.
-  void Create(ClientWindowHandle parent_handle, const CefRect& rect);
-
-  // The below members will only be accessed on the main thread which should be
-  // the same as the CEF UI thread.
-  OsrRenderer renderer_;
-  ClientWindowHandle nsview_;
-  bool hidden_;
-  bool painting_popup_;
+  scoped_ptr<BrowserWindowOsrMacImpl> impl_;
 
   DISALLOW_COPY_AND_ASSIGN(BrowserWindowOsrMac);
+
+  friend class BrowserWindowOsrMacImpl;
 };
 
 }  // namespace client