GCC: do not use anonymous structs

Two using declarations have been added in x11.h for an
unnamed struct. This is not allowed by C++ (and GCC). An example:
using XErrorEvent = struct {
  ...
};

The solution is giving a name to the struct.

Bug: 819294
Change-Id: I9e77017851efe8e0a92fb37c01fdba693bb7a36d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412450Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: José Dapena Paz <jdapena@igalia.com>
Cr-Commit-Position: refs/heads/master@{#807126}

ui/gfx/x/x11.h

@@ -139,7 +139,7 @@ using xcb_connection_t = struct xcb_connection_t;
 
 enum XEventQueueOwner { XlibOwnsEventQueue = 0, XCBOwnsEventQueue };
 
-using XErrorEvent = struct {
+using XErrorEvent = struct _XErrorEvent {
   int type;
   Display* display;
   XID resourceid;
@@ -149,7 +149,7 @@ using XErrorEvent = struct {
   unsigned char minor_code;
 };
 
-using XRectangle = struct {
+using XRectangle = struct _XRectangle {
   short x, y;
   unsigned short width, height;
 };
@@ -161,7 +161,7 @@ using XExtData = struct _XExtData {
   XPointer private_data;
 };
 
-using Visual = struct {
+using Visual = struct _Visual {
   XExtData* ext_data;
   VisualID visualid;
   int c_class;
@@ -170,7 +170,7 @@ using Visual = struct {
   int map_entries;
 };
 
-using XVisualInfo = struct {
+using XVisualInfo = struct _XVisualInfo {
   Visual* visual;
   VisualID visualid;
   int screen;