allow specifyting VS2019 in .mozconfig.

mozilla-central/patches/9999999-with-vs2019.patch

@@ -0,0 +1,40 @@
+
+# HG changeset patch
+# User Bill Gianopoulos <wgianopoulos@gmail.com>
+# Date 1619006020 14400
+#      Wed Apr 21 07:53:40 2021 -0400
+Bug 9999999 - permit --with-visual-studio-version=2019.
+
+diff --git a/build/moz.configure/toolchain.configure b/build/moz.configure/toolchain.configure
+--- a/build/moz.configure/toolchain.configure
++++ b/build/moz.configure/toolchain.configure
+@@ -517,27 +517,26 @@ def get_vc_paths(topsrcdir):
+ def host_is_windows(host):
+     if host.kernel == "WINNT":
+         return True
+ 
+ 
+ option(
+     "--with-visual-studio-version",
+     nargs=1,
+-    choices=("2017",),
++    choices=("2017", "2019",),
+     when=host_is_windows,
+     help="Select a specific Visual Studio version to use",
+ )
+ 
+ 
+ @depends("--with-visual-studio-version", when=host_is_windows)
+ def vs_major_version(value):
+     if value:
+-        return {"2017": 15}[value[0]]
+-
++        return {"2017": 15, "2019": 16}[value[0]]
+ 
+ option(
+     env="VC_PATH",
+     nargs=1,
+     when=host_is_windows,
+     help="Path to the Microsoft Visual C/C++ compiler",
+ )
+ 

mozilla-central/patches/series

@@ -1,2 +1,3 @@
 1624546-vs2019-runtime-mc.patch
 1659298-extensions-moz-v1_1-mc.patch
+9999999-with-vs2019.patch