X11: Fix GPU crash when there's no alpha visuals provided by the system

This fixes a regression introduced after [1].  The issue is that there
may not always be an alpha visual.

[1] https://chromium.googlesource.com/chromium/src/+/76ddcd29071bd4079c8776e0576268da5493f3ff

BUG=1001026
R=kbr

Change-Id: I040082e9e1921c5a541aecb80279ca7c50275f71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906318
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714054}

ui/gl/gl_surface_glx.cc

@@ -459,7 +459,10 @@ bool GLSurfaceGLX::InitializeOneOff() {
     return false;
   }
 
-  const auto& visual_info = gl::GLVisualPickerGLX::GetInstance()->rgba_visual();
+  auto* visual_picker = gl::GLVisualPickerGLX::GetInstance();
+  XVisualInfo visual_info = visual_picker->rgba_visual();
+  if (!visual_info.visual)
+    visual_info = visual_picker->system_visual();
   g_visual = visual_info.visual;
   g_depth = visual_info.depth;
   g_colormap =