[serializer] Fix ExternalReferenceEncoder::NameOfAddress.

This function didn't account for addresses of api-provided external
references, leading to out-of-bound reads on external_reference_table.
(This happened to me when printing a code object in gdb, I'm not sure
how to easily test it.)

Also remove an unused method from the private Value class.

R=jgruber@chromium.org

Change-Id: Id14fed3fb3866df750bcad8f4a02c61748b07ad3
Reviewed-on: https://chromium-review.googlesource.com/1060035Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53195}

src/snapshot/serializer-common.cc

@@ -87,8 +87,9 @@ const char* ExternalReferenceEncoder::NameOfAddress(Isolate* isolate,
                                                     Address address) const {
   Maybe<uint32_t> maybe_index = map_->Get(address);
   if (maybe_index.IsNothing()) return "<unknown>";
-  return isolate->heap()->external_reference_table()->name(
-      maybe_index.FromJust());
+  Value value(maybe_index.FromJust());
+  if (value.is_from_api()) return "<from api>";
+  return isolate->heap()->external_reference_table()->name(value.index());
 }
 
 void SerializedData::AllocateData(uint32_t size) {

src/snapshot/serializer-common.h

@@ -30,7 +30,6 @@ class ExternalReferenceEncoder {
 
     bool is_from_api() const { return IsFromAPI::decode(value_); }
     uint32_t index() const { return Index::decode(value_); }
-    uint32_t raw() const { return value_; }
 
    private:
     class Index : public BitField<uint32_t, 0, 31> {};