Fix BUILD.gn issues for Android & Fuschia

BUG=dawn:287

Change-Id: If39e1211cedefa5d02f1ea2f3ae00f6195e1e916
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/14141
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
diff --git a/third_party/BUILD.gn b/third_party/BUILD.gn
index 1004136..6ecb9cb 100644
--- a/third_party/BUILD.gn
+++ b/third_party/BUILD.gn
@@ -16,8 +16,6 @@
 import("../scripts/dawn_features.gni")
 import("../scripts/dawn_overrides_with_defaults.gni")
 
-is_msvc = is_win && !is_clang
-
 ###############################################################################
 # Third-party dependencies needed by libdawn_native
 ###############################################################################
@@ -166,7 +164,7 @@
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [ "//build/config/compiler:no_chromium_code" ]
 
-    if (is_msvc) {
+    if (is_win && !is_clang) {
       # nonstandard extension, function/data pointer conversion in expression
       cflags_c = [ "/wd4152" ]
     } else {