Move BACKTRACE_NO_TID to BACKTRACE_CURRENT_THREAD.

Use a bit better name for this. The other name was a bit confusing.

Change-Id: I1261f2ee3854a9c8b82133ad0bfbbbe48b43c9ac
diff --git a/libbacktrace/Backtrace.cpp b/libbacktrace/Backtrace.cpp
index b22d301..0348d78 100644
--- a/libbacktrace/Backtrace.cpp
+++ b/libbacktrace/Backtrace.cpp
@@ -214,12 +214,12 @@
 
 Backtrace* Backtrace::Create(pid_t pid, pid_t tid) {
   if (pid == BACKTRACE_CURRENT_PROCESS || pid == getpid()) {
-    if (tid == BACKTRACE_NO_TID || tid == gettid()) {
+    if (tid == BACKTRACE_CURRENT_THREAD || tid == gettid()) {
       return CreateCurrentObj();
     } else {
       return CreateThreadObj(tid);
     }
-  } else if (tid == BACKTRACE_NO_TID) {
+  } else if (tid == BACKTRACE_CURRENT_THREAD) {
     return CreatePtraceObj(pid, pid);
   } else {
     return CreatePtraceObj(pid, tid);