ART: Change iterator value type to match

Make keys const.

Bug: 32619234
Test: m
Change-Id: I240341a0fec79754bea2ed21c0d0a4ddd119ce35
diff --git a/runtime/hprof/hprof.cc b/runtime/hprof/hprof.cc
index 2336759..8cbe491 100644
--- a/runtime/hprof/hprof.cc
+++ b/runtime/hprof/hprof.cc
@@ -574,9 +574,9 @@
   }
 
   void WriteStringTable() {
-    for (const std::pair<std::string, HprofStringId>& p : strings_) {
+    for (const auto& p : strings_) {
       const std::string& string = p.first;
-      const size_t id = p.second;
+      const HprofStringId id = p.second;
 
       output_->StartNewRecord(HPROF_TAG_STRING, kHprofTime);