[XProto] Check that the Xlib event queue is empty on destruction

The event queue is never cleared, so if any events are requested,
they'll leak memory.  This CL adds a check to ensure no events were
ever pending.  It's added as a CHECK(), not a DCHECK(), since we
want to receive crash reports if users with untested GPU drivers
ever hit this.

Change-Id: I2aa2fd9f88a1637deff5e81996de71c1d5956e0f
Bug: 1158170
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2602704
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840705}

ui/gfx/x/BUILD.gn

@@ -36,6 +36,7 @@ generate_library_loader("xlib_loader") {
     "XSynchronize",
     "XSetErrorHandler",
     "XFree",
+    "XPending",
   ]
 }
 

ui/gfx/x/xlib.h

@@ -13,6 +13,7 @@ int XFlush(struct _XDisplay*);
 int XSynchronize(struct _XDisplay*, int);
 int XSetErrorHandler(int (*)(void*, void*));
 void XFree(void*);
+int XPending(struct _XDisplay*);
 }
 
 #endif  // UI_GFX_X_XLIB_H_

ui/gfx/x/xlib_support.cc

@@ -73,8 +73,15 @@ XlibDisplay::XlibDisplay(const std::string& address) {
 
 DISABLE_CFI_ICALL
 XlibDisplay::~XlibDisplay() {
-  if (display_)
-    GetXlibLoader()->XCloseDisplay(display_);
+  if (!display_)
+    return;
+
+  auto* loader = GetXlibLoader();
+  // Events are not processed on |display_|, so if any client asks to receive
+  // events, they will just queue up and leak memory.  This check makes sure
+  // |display_| never had any pending events before it is closed.
+  CHECK(!loader->XPending(display_));
+  loader->XCloseDisplay(display_);
 }
 
 DISABLE_CFI_ICALL