DisplayWGL: Allow MakeCurrent(null).

Previously we would ignore unsetting calls to MakeCurrent. But this
would prevent ANGLE from unsetting driver state on the current thread.

Fix this by checking for 0/null explicitly in DisplayWGL::makeCurrent.

This turned up when running test configs without recreating the
Display.

Bug: angleproject:3261
Change-Id: I7f747dc7649a06f019f1b89655cb71dd3b151aa0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1520992
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>

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

@@ -641,13 +641,7 @@ egl::Error DisplayWGL::initializeD3DDevice()
         return egl::EglNotInitialized() << "Could not create D3D11 device, " << gl::FmtHR(result);
     }
 
-    egl::Error error = registerD3DDevice(mD3D11Device, &mD3D11DeviceHandle);
-    if (error.isError())
-    {
-        return error;
-    }
-
-    return egl::NoError();
+    return registerD3DDevice(mD3D11Device, &mD3D11DeviceHandle);
 }
 
 void DisplayWGL::generateExtensions(egl::DisplayExtensions *outExtensions) const
@@ -709,6 +703,10 @@ egl::Error DisplayWGL::makeCurrent(egl::Surface *drawSurface,
         SurfaceWGL *drawSurfaceWGL = GetImplAs<SurfaceWGL>(drawSurface);
         newDC                      = drawSurfaceWGL->getDC();
     }
+    else
+    {
+        newDC = mDeviceContext;
+    }
 
     HGLRC newContext = currentContext.glrc;
     if (context)
@@ -716,9 +714,15 @@ egl::Error DisplayWGL::makeCurrent(egl::Surface *drawSurface,
         ContextWGL *contextWGL = GetImplAs<ContextWGL>(context);
         newContext             = contextWGL->getContext();
     }
+    else
+    {
+        newContext = 0;
+    }
 
     if (newDC != currentContext.dc || newContext != currentContext.glrc)
     {
+        ASSERT(newDC != 0);
+
         if (!mFunctionsWGL->makeCurrent(newDC, newContext))
         {
             // TODO(geofflang): What error type here?

src/tests/egl_tests/EGLQueryContextTest.cpp

@@ -51,10 +51,14 @@ class EGLQueryContextTest : public EGLTest, public testing::WithParamInterface<P
 
     void TearDown() override
     {
-        eglMakeCurrent(mDisplay, EGL_NO_SURFACE, EGL_NO_SURFACE, EGL_NO_CONTEXT);
-        eglDestroyContext(mDisplay, mContext);
-        eglDestroySurface(mDisplay, mSurface);
-        eglTerminate(mDisplay);
+        if (mDisplay != EGL_NO_DISPLAY)
+        {
+            eglMakeCurrent(mDisplay, EGL_NO_SURFACE, EGL_NO_SURFACE, EGL_NO_CONTEXT);
+            eglDestroyContext(mDisplay, mContext);
+            eglDestroySurface(mDisplay, mSurface);
+            eglTerminate(mDisplay);
+        }
+        ASSERT_EGL_SUCCESS() << "Error during test TearDown";
     }
 
     EGLDisplay mDisplay;
@@ -102,6 +106,7 @@ TEST_P(EGLQueryContextTest, GetRenderBufferBoundSurface)
     EXPECT_TRUE(eglQueryContext(mDisplay, mContext, EGL_RENDER_BUFFER, &contextRenderBuffer) !=
                 EGL_FALSE);
     EXPECT_TRUE(renderBuffer == contextRenderBuffer);
+    ASSERT_EGL_SUCCESS();
 }
 
 TEST_P(EGLQueryContextTest, BadDisplay)