Use less error prone string list, map or multimap pointer types

Use pointers to incomplete struct types. If an argument of a not suitable
type is passed to a function of any string collection,
C compiler will warn:
  warning: passing argument 1 of <func> from incompatible pointer type
C++ compiler will raise an error, for example:
  error: cannot convert 'cef_string_list_t' ... to 'cef_string_multimap_t'

With the previously used `void*` pointer types, such errors in a code were not
diagnosed.

include/internal/cef_string_list.h

@@ -41,7 +41,7 @@ extern "C" {
 ///
 /// CEF string maps are a set of key/value string pairs.
 ///
-typedef void* cef_string_list_t;
+typedef struct _cef_string_list_t* cef_string_list_t;
 
 ///
 /// Allocate a new string map.

include/internal/cef_string_map.h

@@ -41,7 +41,7 @@ extern "C" {
 ///
 /// CEF string maps are a set of key/value string pairs.
 ///
-typedef void* cef_string_map_t;
+typedef struct _cef_string_map_t* cef_string_map_t;
 
 ///
 /// Allocate a new string map.

include/internal/cef_string_multimap.h

@@ -42,7 +42,7 @@ extern "C" {
 /// CEF string multimaps are a set of key/value string pairs.
 /// More than one value can be assigned to a single key.
 ///
-typedef void* cef_string_multimap_t;
+typedef struct _cef_string_multimap_t* cef_string_multimap_t;
 
 ///
 /// Allocate a new string multimap.

libcef/common/string_list_impl.cc

@@ -13,7 +13,7 @@ using StringList = std::vector<CefString>;
 }  // namespace
 
 CEF_EXPORT cef_string_list_t cef_string_list_alloc() {
-  return new StringList;
+  return reinterpret_cast<cef_string_list_t>(new StringList);
 }
 
 CEF_EXPORT size_t cef_string_list_size(cef_string_list_t list) {
@@ -58,5 +58,5 @@ CEF_EXPORT void cef_string_list_free(cef_string_list_t list) {
 CEF_EXPORT cef_string_list_t cef_string_list_copy(cef_string_list_t list) {
   DCHECK(list);
   StringList* impl = reinterpret_cast<StringList*>(list);
-  return new StringList(*impl);
+  return reinterpret_cast<cef_string_list_t>(new StringList(*impl));
 }

libcef/common/string_map_impl.cc

@@ -13,7 +13,7 @@ using StringMap = std::map<CefString, CefString>;
 }  // namespace
 
 CEF_EXPORT cef_string_map_t cef_string_map_alloc() {
-  return new StringMap;
+  return reinterpret_cast<cef_string_map_t>(new StringMap);
 }
 
 CEF_EXPORT size_t cef_string_map_size(cef_string_map_t map) {

libcef/common/string_multimap_impl.cc

@@ -13,7 +13,7 @@ using StringMultimap = std::multimap<CefString, CefString>;
 }  // namespace
 
 CEF_EXPORT cef_string_multimap_t cef_string_multimap_alloc() {
-  return new StringMultimap;
+  return reinterpret_cast<cef_string_multimap_t>(new StringMultimap);
 }
 
 CEF_EXPORT size_t cef_string_multimap_size(cef_string_multimap_t map) {