Try to fix msan bot after libc++ roll.

Should fix:
../../buildtools/third_party/libc++/trunk/include/map:843:5: error: static_assert failed
    "Allocator::value_type must be same type as value_type"
    static_assert((is_same<typename allocator_type::value_type, value_type>::value),
    ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../components/metrics/leak_detector/leak_analyzer.h:69:55: note: requested here
           Allocator<std::pair<ValueType, uint32_t>>> suspected_histogram_;

BUG=573250

Review URL: https://codereview.chromium.org/1557663002 .

Cr-Commit-Position: refs/heads/master@{#367196}

components/metrics/leak_detector/leak_analyzer.h

@@ -66,7 +66,8 @@ class LeakAnalyzer {
   std::map<ValueType,
            uint32_t,
            std::less<ValueType>,
-           Allocator<std::pair<ValueType, uint32_t>>> suspected_histogram_;
+           Allocator<std::pair<const ValueType, uint32_t>>>
+      suspected_histogram_;
 
   // Array of allocated values that passed the suspicion threshold and are being
   // reported.