Linux: Add support for ozone builds (fixes issue #2296).

Ozone builds can run with different platform backends (Wayland, X11, etc). Usage of the Views framework is required, and the cefclient sample application is not supported.

Example usage:

$ export GN_DEFINES="use_ozone=true"
$ cd /path/to/chromium/src/cef
$ ./cef_create_projects.sh
$ cd /path/to/chromium/src
$ ninja -C out/Release_GN_x64 cefsimple
$ ./out/Release_GN_x64/cefsimple --use-views --ozone-platform=wayland

Binary distributions can be created by passing the `--ozone` flag to make_distrib.py.

libcef/browser/native/browser_platform_delegate_native_linux.h

@@ -7,7 +7,9 @@
 
 #include "libcef/browser/native/browser_platform_delegate_native.h"
 
+#if defined(USE_X11)
 class CefWindowX11;
+#endif
 
 // Windowed browser implementation for Linux.
 class CefBrowserPlatformDelegateNativeLinux
@@ -60,7 +62,9 @@ class CefBrowserPlatformDelegateNativeLinux
   // associated root window is destroyed.
   views::Widget* window_widget_;
 
-  CefWindowX11* window_x11_;
+#if defined(USE_X11)
+  CefWindowX11* window_x11_ = nullptr;
+#endif
 };
 
 #endif  // CEF_LIBCEF_BROWSER_NATIVE_BROWSER_PLATFORM_DELEGATE_NATIVE_LINUX_H_