Change some kNone to kUnknown.

Missed a couple kNone settings in the Tint executable when attempting
to determine the output format. This CL updates the to be kUnknown.

Change-Id: Ia0c4293fe69711cf1de878255bd18c6eeec4bffe
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/110502
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
diff --git a/src/tint/cmd/main.cc b/src/tint/cmd/main.cc
index 4a6578f..4eababd 100644
--- a/src/tint/cmd/main.cc
+++ b/src/tint/cmd/main.cc
@@ -95,7 +95,7 @@
 
     std::unordered_set<uint32_t> skip_hash;
 
-    Format format = Format::kNone;
+    Format format = Format::kUnknown;
 
     bool emit_single_entry_point = false;
     std::string ep_name;
@@ -246,7 +246,7 @@
     }
 #endif  // TINT_BUILD_HLSL_WRITER
 
-    return Format::kNone;
+    return Format::kUnknown;
 }
 
 std::vector<std::string> split_on_char(std::string list, char c) {