[snapshot][cleanup] Using 'using' instead of 'typedef'

Even though both are allowed in the style guide, it recommends to use
'using', as its syntax is more consistent with the rest of C++.
This CL turns all typedefs in src/snapshot to 'using' declarations.

R=jgruber@chromium.org

Bug: v8:8834
Change-Id: Ie555e9ac7e1ec04c20d411647f8ab70f671c1cb2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1545903
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60699}

src/snapshot/mksnapshot.cc

@@ -246,7 +246,7 @@ void WriteEmbeddedFile(i::EmbeddedFileWriter* writer) {
   writer->WriteEmbedded(&embedded_blob);
 }
 
-typedef std::map<std::string, int> CounterMap;
+using CounterMap = std::map<std::string, int>;
 CounterMap* counter_map_ = nullptr;
 
 void MaybeSetCounterFunction(v8::Isolate* isolate) {

src/snapshot/natives.h

@@ -39,8 +39,7 @@ class V8_EXPORT_PRIVATE NativesCollection {
   static Vector<const char> GetScriptsSource();
 };
 
-typedef NativesCollection<EXTRAS> ExtraNatives;
-
+using ExtraNatives = NativesCollection<EXTRAS>;
 
 #ifdef V8_USE_EXTERNAL_STARTUP_DATA
 // Used for reading the natives at runtime. Implementation in natives-empty.cc

src/snapshot/references.h

@@ -154,7 +154,7 @@ class SerializerReferenceMap
                                        base::KeyEqualityMatcher<intptr_t>,
                                        base::DefaultAllocationPolicy> {
  public:
-  typedef base::TemplateHashMapEntry<uintptr_t, SerializerReference> Entry;
+  using Entry = base::TemplateHashMapEntry<uintptr_t, SerializerReference>;
 
   SerializerReferenceMap() : attached_reference_index_(0) {}