update skstd

- Lots of skstd::foo is now std::foo since C++14.
- Get rid of SK_WHEN(cond,T); std::enable_if_t<cond,T> is pithy enough.
- Move SkBitmaskEnum.h contents into sknonstd.

Change-Id: Ie5dc459405b1ff55e5b3ac57e70df7edd7cf38c0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/286315
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/tools/skui/ModifierKey.h b/tools/skui/ModifierKey.h
index 09c5536..ffb358c 100644
--- a/tools/skui/ModifierKey.h
+++ b/tools/skui/ModifierKey.h
@@ -16,7 +16,7 @@
 };
 }
 
-namespace skstd {
+namespace sknonstd {
 template <> struct is_bitmask_enum<skui::ModifierKey> : std::true_type {};
 }
 #endif  // skui_modifierkey_defined