win: Fix two -Wmicrosoft-cast warnings.

The C++ standard doesn't allow implicit conversion of function pointers
(such as returned by GetProcAddress()) to void*.  MSVC does accept that,
and so does clang-cl (albeit with a warning).  To fix the warning,
just make LoadComBaseFunction() return FARPROC.

Bug: 550065
Change-Id: I95eeed86c31d45358aabc31414435ae79a1a0e5c
Reviewed-on: https://chromium-review.googlesource.com/955905Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541872}

base/win/core_winrt_util.cc

@@ -6,7 +6,7 @@
 
 namespace {
 
-void* LoadComBaseFunction(const char* function_name) {
+FARPROC LoadComBaseFunction(const char* function_name) {
   static HMODULE const handle = ::LoadLibrary(L"combase.dll");
   return handle ? ::GetProcAddress(handle, function_name) : nullptr;
 }

base/win/scoped_hstring.cc

@@ -15,7 +15,7 @@ namespace {
 
 static bool g_load_succeeded = false;
 
-void* LoadComBaseFunction(const char* function_name) {
+FARPROC LoadComBaseFunction(const char* function_name) {
   static HMODULE const handle = ::LoadLibrary(L"combase.dll");
   return handle ? ::GetProcAddress(handle, function_name) : nullptr;
 }