Linux perf support update.

src/base/platform/platform-posix.cc

Index: src/base/platform/platform-posix.cc
diff --git a/src/base/platform/platform-posix.cc b/src/base/platform/platform-posix.cc
index 8454213def9308ef2d89e9b758f506af764e330c..75283d52ef73f4e0b673dd1a88579b47efe05681 100644
--- a/src/base/platform/platform-posix.cc
+++ b/src/base/platform/platform-posix.cc
@@ -20,9 +20,9 @@
 #include <sys/mman.h>
 #include <sys/resource.h>
 #include <sys/stat.h>
+#include <sys/syscall.h>
 #include <sys/time.h>
 #include <sys/types.h>
-
 #if defined(__linux__)
 #include <sys/prctl.h>  // NOLINT, for prctl
 #endif
@@ -318,6 +318,11 @@ int OS::GetCurrentProcessId() {
 }
 
 
+int OS::GetCurrentThreadId() {
+  return static_cast<int>(syscall(SYS_gettid));
+}
+
+
 // ----------------------------------------------------------------------------
 // POSIX date/time support.
 //