Fix -Wloop-analysis warnings in pdf/.

These indicate when a range-based for loop is using an index whose type
(value, pointer, or reference) doesn't match what the loop actually
extracts from the range.  Fix by matching the actual type better.

This shouldn't cause any behavior/performance change, just be slightly
clearer about what's actually happening when reading the code.

Bug: 1223264
Change-Id: Ib95f240cc46dc949e6c2501de8f29ee02321878e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2984973
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#896117}

pdf/ppapi_migration/value_conversions.cc

@@ -36,7 +36,7 @@ pp::Var VarFromValue(const base::Value& value) {
     }
     case base::Value::Type::DICTIONARY: {
       pp::VarDictionary var_dict;
-      for (const auto& value_pair : value.DictItems()) {
+      for (auto value_pair : value.DictItems()) {
         var_dict.Set(value_pair.first, VarFromValue(value_pair.second));
       }
       return var_dict;