Reuse surfaceless GPU connection when re-entering WebVR

https://crrev.com/c/1120677 "Don't create a Surface for WebXR SharedBuffer
mode" had a logic error that led to the GPU process connection being recreated
unconditionally on re-entering VR, resulting in an inconsistent state where it
tried to reuse mailbox names belonging to the previous connection.

Change-Id: Ic005c7884a46b3d1b3da986a55e9d733f09d68d3
Reviewed-on: https://chromium-review.googlesource.com/1125273Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572390}

chrome/browser/android/vr/vr_shell_gl.cc

@@ -859,7 +859,11 @@ void VrShellGl::ConnectPresentingService(
       GetWebVrFrameTransportOptions(options);
 
   if (webxr_use_shared_buffer_draw_) {
-    CreateSurfaceBridge(nullptr);
+    // Create the mailbox bridge if it doesn't exist yet. We can continue
+    // reusing the existing one if it does, its resources such as mailboxes
+    // are still valid.
+    if (!mailbox_bridge_)
+      CreateSurfaceBridge(nullptr);
   } else {
     if (!webvr_surface_texture_) {
       webvr_surface_texture_ = gl::SurfaceTexture::Create(webvr_texture_id_);