ui: Enable virtual contexts for linux

ui/gl/gl_surface_glx.h

Index: ui/gl/gl_surface_glx.h
diff --git a/ui/gl/gl_surface_glx.h b/ui/gl/gl_surface_glx.h
index c1fca6006a1726b5971bd0494d284220ad61e22d..46b044fa7993c460a0486d6c38774d79ccc50982 100644
--- a/ui/gl/gl_surface_glx.h
+++ b/ui/gl/gl_surface_glx.h
@@ -44,6 +44,8 @@ class GL_EXPORT GLSurfaceGLX : public GLSurface {
  protected:
   ~GLSurfaceGLX() override;
 
+  static void* GetConfig(gfx::AcceleratedWidget window);
+
  private:
   DISALLOW_COPY_AND_ASSIGN(GLSurfaceGLX);
 };
@@ -93,9 +95,9 @@ class GL_EXPORT NativeViewGLSurfaceGLX : public GLSurfaceGLX,
 };
 
 // A surface used to render to an offscreen pbuffer.
-class GL_EXPORT PbufferGLSurfaceGLX : public GLSurfaceGLX {
+class GL_EXPORT UnmappedNativeViewGLSurfaceGLX : public GLSurfaceGLX {
  public:
-  explicit PbufferGLSurfaceGLX(const gfx::Size& size);
+  explicit UnmappedNativeViewGLSurfaceGLX(const gfx::Size& size);
 
   // Implement GLSurfaceGLX.
   bool Initialize() override;
@@ -107,14 +109,15 @@ class GL_EXPORT PbufferGLSurfaceGLX : public GLSurfaceGLX {
   void* GetConfig() override;
 
  protected:
-  ~PbufferGLSurfaceGLX() override;
+  ~UnmappedNativeViewGLSurfaceGLX() override;
 
  private:
   gfx::Size size_;
   void* config_;
-  XID pbuffer_;
+  // Unmapped dummy window, used to provide a compatible surface.
+  gfx::AcceleratedWidget window_;
 
-  DISALLOW_COPY_AND_ASSIGN(PbufferGLSurfaceGLX);
+  DISALLOW_COPY_AND_ASSIGN(UnmappedNativeViewGLSurfaceGLX);
 };
 
 }  // namespace gfx