Fix remaining -pedantic warnings.

Wgnu-zero-variadic-macro-arguments is fixed by:

 - A previous googletest roll fixing the warning in gmock.
 - Adding a dummy argument to AddTraceEvent so that the __VA_ARGS__ is
never empty in TraceEvent.h and doesn't require __VA_ARGS__ token
pasting with a comma.
 - Extracting the first parameter in DAWN_INSTANTIATE_TEST with some
preprocessor tricks instead of singling it out, to avoid __VA_ARGS__
token pasting with a comma.

Wmicrosoft-enum-value is fixed by a previous spirv-cross roll that fixes
the warning upstream.

Bug: dawn:394
Change-Id: Icfe037ae9549087e9d62b6f42f91958addbb53ee
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/21483
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/common/BUILD.gn

@@ -106,23 +106,17 @@ config("dawn_internal") {
       "-Wtautological-unsigned-zero-compare",
     ]
 
-    # clang-cl doesn't know -pedantic, pass it explicitly to the clang driver on Windows
     if (is_win) {
-      cflags += [ "/clang:-pedantic" ]
+      cflags += [
+        # clang-cl doesn't know -pedantic, pass it explicitly to the clang driver
+        "/clang:-pedantic",
+
+        # Allow the use of __uuidof()
+        "-Wno-language-extension-token",
+      ]
     } else {
       cflags += [ "-pedantic" ]
     }
-
-    # On Windows allow the use of __uuidof()
-    if (is_win) {
-      cflags += [ "-Wno-language-extension-token" ]
-    }
-
-    # Flags for -pedantic warnings that haven't been fixed yet.
-    cflags += [
-      "-Wno-gnu-zero-variadic-macro-arguments",
-      "-Wno-microsoft-enum-value",
-    ]
   }
 
   if (!is_clang && is_win) {