Fixup unused function and variable.

This CL fixes build issues seen on the fuzzers when they disable
logging.

Bug: chromium:1337052
Change-Id: I91f6853a7550ccbffb924a5fb65cd89086f78853
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/94241
Reviewed-by: Ben Clayton <bclayton@google.com>
Auto-Submit: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/dawn/common/Log.cpp

@@ -28,6 +28,7 @@ namespace dawn {
 
 namespace {
 
+#if !defined(DAWN_DISABLE_LOGGING)
 const char* SeverityName(LogSeverity severity) {
     switch (severity) {
         case LogSeverity::Debug:
@@ -43,6 +44,7 @@ const char* SeverityName(LogSeverity severity) {
             return "";
     }
 }
+#endif
 
 #if DAWN_PLATFORM_IS(ANDROID)
 android_LogPriority AndroidLogPriority(LogSeverity severity) {
@@ -72,6 +74,7 @@ LogMessage& LogMessage::operator=(LogMessage&& other) = default;
 
 #if defined(DAWN_DISABLE_LOGGING)
 LogMessage::~LogMessage() {
+    (void)mSeverity;
     // Don't print logs to make fuzzing more efficient. Implemented as
     // an early return to avoid warnings about unused member variables.
     return;