Don't set a null current WGL context in the Surface destructor.

By setting a null current context, there is a brief window where all GL
calls will fail.  Since there is only one WGL context, just leave in on
the unreferenced window until a new window is made current.

BUG=angleproject:890

Change-Id: I51eadf23ca61e274f7d174ac5a9e0592bcdadebc
Reviewed-on: https://chromium-review.googlesource.com/265483
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>

src/libANGLE/renderer/gl/wgl/WindowSurfaceWGL.cpp

@@ -33,7 +33,6 @@ WindowSurfaceWGL::~WindowSurfaceWGL()
     mPixelFormat = 0;
     mShareWGLContext = nullptr;
 
-    mFunctionsWGL->makeCurrent(mChildDeviceContext, nullptr);
     ReleaseDC(mChildWindow, mChildDeviceContext);
     mChildDeviceContext = nullptr;