Move DumpBacktrace() to checks.cc and cleanup both the code and the necessary platform checks.

src/platform.h

Index: src/platform.h
diff --git a/src/platform.h b/src/platform.h
index e0e62fa0237d6581e586a4d7098810986311266a..527de1606c4453c41a5f2e8cb877b073ae510d91 100644
--- a/src/platform.h
+++ b/src/platform.h
@@ -256,9 +256,6 @@ class OS {
   // Debug break.
   static void DebugBreak();
 
-  // Dump C++ current stack trace (only functional on Linux).
-  static void DumpBacktrace();
-
   // Walk the stack.
   static const int kStackWalkError = -1;
   static const int kStackWalkMaxNameLen = 256;