PlatformWindow: Add X11Extension.

As stated in https://crrev.com/c/1917765, to provide more
flexibility and avoid casts to PlatformWindowLinux, the extension
interfaces were added instead. This fixes the crash when
Ozone for Linux is run with --ozone-platform=headless.

Please note that the most x11 specific interfaces will
be removed as soon as there are no callers for them.

Bug: 1016556
Change-Id: I90591fc6e2e2fd776685384a05cc3f9bdd770fc3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917536
Commit-Queue: Maksim Sisov <[email protected]>
Reviewed-by: Scott Violet <[email protected]>
Reviewed-by: Robert Kroeger <[email protected]>
Reviewed-by: Thomas Anderson <[email protected]>
Cr-Commit-Position: refs/heads/master@{#718615}
63 files changed