RefBase: remove implicit conversion to bool.

This helps push for comparing against nullptr more consistently.

Also replaces .Get() == nullptr and .Get() != nullptr with just ==
nullptr and != nullptr.

Bug: dawn:89
Change-Id: I884a4819f97305a73c11bad84391d1d2113ab7e2
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/32922
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Stephen White <senorblanco@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/null/DeviceNull.cpp

@@ -387,7 +387,7 @@ namespace dawn_native { namespace null {
     }
 
     void SwapChain::DetachFromSurfaceImpl() {
-        if (mTexture.Get() != nullptr) {
+        if (mTexture != nullptr) {
             mTexture->Destroy();
             mTexture = nullptr;
         }