[XProto] Fix startup crash on systems missing XInput

This CL includes 2 event fixes that were originally part of
CL [1] to fix similar event-related issues.

* event.h
    - "As() const" always called back into itself, causing an
      infinite loop.  The intention was to call the non-const
      As() function.
* event.cc
    - Xlib modifies the event data when parsing some events,
      but we never do, so let us parse the event before Xlib.
* gen_xproto.py
    - Ensure the extension is present before creating extension
      events.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2253223/

BUG=1098194, 1098204
R=thestig

Change-Id: I414acd9c138a92fbddd09da705127f1d7f60b552
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2261177
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781615}

ui/gfx/x/event.cc

@@ -46,6 +46,10 @@ Event::Event(xcb_generic_event_t* xcb_event,
     }
   }
 
+  // Xlib sometimes modifies |xcb_event|, so let it handle the event after
+  // we parse it with ReadEvent().
+  ReadEvent(this, connection, reinterpret_cast<uint8_t*>(xcb_event));
+
   _XEnq(display, reinterpret_cast<xEvent*>(xcb_event));
   if (!XEventsQueued(display, QueuedAlready)) {
     // If Xlib gets an event it doesn't recognize (eg. from an
@@ -60,8 +64,6 @@ Event::Event(xcb_generic_event_t* xcb_event,
   XNextEvent(display, &xlib_event_);
   if (xlib_event_.type == x11::GeGenericEvent::opcode)
     XGetEventData(display, &xlib_event_.xcookie);
-
-  ReadEvent(this, connection, reinterpret_cast<uint8_t*>(xcb_event));
 }
 
 Event::Event(Event&& event) {

ui/gfx/x/event.h

@@ -57,7 +57,7 @@ class COMPONENT_EXPORT(X11) Event {
 
   template <typename T>
   const T* As() const {
-    return const_cast<const Event*>(this)->As<T>();
+    return const_cast<Event*>(this)->As<T>();
   }
 
   bool sequence_valid() const { return sequence_valid_; }

ui/gfx/x/gen_xproto.py

@@ -1460,12 +1460,14 @@ class GenReadEvent(FileWriter):
             # GenericEvent extension event
             conds.extend([
                 'evtype == GeGenericEvent::opcode',
+                '%s.present()' % ext,
                 'ge->extension == %s.major_opcode()' % ext,
             ])
             opcode = 'ge->event_type'
         else:
             # Extension event
             opcode = 'evtype - %s.first_event()' % ext
+            conds.append('%s.present()' % ext)
 
         if len(event.opcodes) == 1:
             conds.append('%s == %s::opcode' % (opcode, typename))