base: Add const to a few PlatformThreadHandle methods

base/threading/platform_thread.h

Index: base/threading/platform_thread.h
diff --git a/base/threading/platform_thread.h b/base/threading/platform_thread.h
index 9742d570c6fb7def3dbf6130769182b962afa04d..f2f3b260586cdcf67a75fae4b0a3e0119ece61f6 100644
--- a/base/threading/platform_thread.h
+++ b/base/threading/platform_thread.h
@@ -53,15 +53,15 @@ class PlatformThreadHandle {
         id_(id) {
   }
 
-  bool is_equal(const PlatformThreadHandle& other) {
+  bool is_equal(const PlatformThreadHandle& other) const {
     return handle_ == other.handle_;
   }
 
-  bool is_null() {
+  bool is_null() const {
     return !handle_;
   }
 
-  Handle platform_handle() {
+  Handle platform_handle() const {
     return handle_;
   }