Dawn: silence some warnings on win-clang.

Change-Id: Ifcab9738b5e1d17ed71e26fa30bb34951e421567
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/249126
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
diff --git a/tools/gpu/dawn/DawnTestContext.cpp b/tools/gpu/dawn/DawnTestContext.cpp
index 666ea6c..97ed871 100644
--- a/tools/gpu/dawn/DawnTestContext.cpp
+++ b/tools/gpu/dawn/DawnTestContext.cpp
@@ -62,10 +62,10 @@
 private:
     Proc getProc(const char* name) {
         PROC proc;
-        if (proc = GetProcAddress(fModule, name)) {
+        if ((proc = GetProcAddress(fModule, name))) {
             return (Proc) proc;
         }
-        if (proc = wglGetProcAddress(name)) {
+        if ((proc = wglGetProcAddress(name))) {
             return (Proc) proc;
         }
         return nullptr;