[XProto] Replace XAtom with x11::Atom

This CL removes XAtom from x11_types.h and replaces all usages with
x11::Atom.  This means that ui/gfx/x/connection.h needs to be included
everywhere x11_types.h was included, so it's just included from
x11_types.h directly.

This causes a problem: xproto_undef.h undefines a bunch of Xlib macros
like CopyFromParent, so these usages are replaced with "0 /*
CopyFromParent */" until further refactoring.  There are a couple of
cases where macros are not undefined (eg. Bool), so direct inclusions
of ui/gfx/x/x11.h are added to fix these.

Unfortunately, this CL introduces a lot of ugly static_cast's to
convert from x11::Atom to uint32_t for Xlib APIs, which will be
removed as usage of these Xlib APIs are removed.

I also fixed any clang-tidy warnings that showed up in my editor along
the way.

BUG=1066670
R=sky

Change-Id: Ifdc3c1a137a39cbbb37407e4299026c70da5281c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2202789
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#769975}