[vm/build] Update WINVER to match NTDDI_VERSION

We bumped NTDDI_VERSION to Windows 10 but have not updated WINVER in 
the same way making them diverge for no good reason.

FWIW NTDDI_VERSION should probably be set by winver config, but I am 
not going to fix that now.

Change-Id: I53cdf1cde40aa50911aa1311ca15705d6a6a6964
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/471181
Commit-Queue: Slava Egorov <vegorov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>

build/config/win/BUILD.gn

@@ -14,7 +14,7 @@ config("sdk") {
     "_ATL_NO_OPENGL",
     "_WINDOWS",
     "CERT_CHAIN_PARA_HAS_EXTRA_FIELDS",
-    "NTDDI_VERSION=0x0A000006",
+    "NTDDI_VERSION=0x0A000006",  # NTDDI_WIN10_RS5
     "PSAPI_VERSION=1",
     "WIN32",
     "_SECURE_ATL",
@@ -28,8 +28,8 @@ config("sdk") {
 # targets need to manually override it for their compiles.
 config("winver") {
   defines = [
-    "_WIN32_WINNT=0x0603",
-    "WINVER=0x0603",
+    "_WIN32_WINNT=0x0A00",  # _WIN32_WINNT_WIN10
+    "WINVER=0x0A00",  # _WIN32_WINNT_WIN10
   ]
 }