ui/gl: Initialize |offscreen_surface_| in GLSurfaceEGLSurfaceControl.

R=piman@chromium.org

Bug: 958981
Change-Id: I28cc5808f02c17b372e0557652c188ae1e2b9a27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1632899
Commit-Queue: Khushal <khushalsagar@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Auto-Submit: Khushal <khushalsagar@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663977}

ui/gl/gl_surface_egl_surface_control.h

@@ -193,7 +193,7 @@ class GL_EXPORT GLSurfaceEGLSurfaceControl : public GLSurfaceEGL {
   bool transaction_ack_pending_ = false;
 
   gfx::OverlayTransform display_transform_ = gfx::OVERLAY_TRANSFORM_NONE;
-  EGLSurface offscreen_surface_;
+  EGLSurface offscreen_surface_ = nullptr;
   base::CancelableOnceClosure check_pending_presentation_callback_queue_task_;
 
   scoped_refptr<base::SingleThreadTaskRunner> gpu_task_runner_;