Consolidate loggability checks and replace streams.

Currently we check if a message should be printed at the call site using LogMessage::Loggable, in the LogMessage itself using LogMessage::IsNoop and in LogMessage::OutputToDebug using log_to_stderr_.

This change unifies the first two of these into a early return in Log().

Bug: webrtc:8982
Change-Id: Ia4e3e12b34716d76c05807e44db1ed4a62dffb87
Reviewed-on: https://webrtc-review.googlesource.com/97440
Commit-Queue: Jonas Olsson <jonasolsson@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24547}

rtc_base/stringutils.cc

@@ -146,4 +146,10 @@ std::string ToHex(const int i) {
   return std::string(buffer);
 }
 
+std::string LeftPad(char padding, unsigned length, std::string s) {
+  if (s.length() >= length)
+    return s;
+  return std::string(length - s.length(), padding) + s;
+}
+
 }  // namespace rtc