Bug 1543501 - Make MOZ_DBG() work on Android. r=heycam

stderr does not go anywhere by default on Android. Debugging output is
conventionally sent to the Android log via special functions like
__android_log_print.

We already have logic for handling this in nsCRTGlue's printf_stderr, but
I don't think we can use that in MFBT.

Differential Revision: https://phabricator.services.mozilla.com/D26985

--HG--
extra : moz-landing-system : lando

mfbt/DbgMacro.h

@@ -18,6 +18,10 @@
 template <typename T>
 class nsTSubstring;
 
+#ifdef ANDROID
+#  include <android/log.h>
+#endif
+
 namespace mozilla {
 
 namespace detail {
@@ -92,7 +96,11 @@ auto&& MozDbg(const char* aFile, int aLine, const char* aExpression,
   s << "[MozDbg] [" << aFile << ':' << aLine << "] " << aExpression << " = ";
   mozilla::DebugValue(s, std::forward<T>(aValue));
   s << '\n';
+#ifdef ANDROID
+  __android_log_print(ANDROID_LOG_INFO, "Gecko", "%s", s.str().c_str());
+#else
   fputs(s.str().c_str(), stderr);
+#endif
   return std::forward<T>(aValue);
 }