Convert LogMessage to explicit.

This CL adds explicit and fixes up the various call sites for the
LogMessage class.

Bug: dawn:1339
Change-Id: Id244d0e31929d31346af48e116a40e7079a50372
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/86872
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>

src/dawn/common/Log.cpp

@@ -92,19 +92,19 @@ namespace dawn {
     }
 
     LogMessage DebugLog() {
-        return {LogSeverity::Debug};
+        return LogMessage(LogSeverity::Debug);
     }
 
     LogMessage InfoLog() {
-        return {LogSeverity::Info};
+        return LogMessage(LogSeverity::Info);
     }
 
     LogMessage WarningLog() {
-        return {LogSeverity::Warning};
+        return LogMessage(LogSeverity::Warning);
     }
 
     LogMessage ErrorLog() {
-        return {LogSeverity::Error};
+        return LogMessage(LogSeverity::Error);
     }
 
     LogMessage DebugLog(const char* file, const char* function, int line) {

src/dawn/common/Log.h

@@ -59,7 +59,7 @@ namespace dawn {
     // Essentially an ostringstream that will print itself in its destructor.
     class LogMessage {
       public:
-        LogMessage(LogSeverity severity);
+        explicit LogMessage(LogSeverity severity);
         ~LogMessage();
 
         LogMessage(LogMessage&& other) = default;