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/common/RefBase.h

@@ -134,10 +134,6 @@ class RefBase {
         return mValue != other;
     }
 
-    operator bool() {
-        return mValue != kNullValue;
-    }
-
     const T operator->() const {
         return mValue;
     }