gn win: exclude sse4 .S files, not used on Windows

It took me far too long to realize, but in gyp .S files are silently
ignored (even though they're added to sources in this case).

R=brettw@chromium.org
TBR=thakis@chromium.org
BUG=354261

Review URL: https://codereview.chromium.org/431733002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@286679 0039d316-1c4b-4281-b951-d872f2087c98

skia/BUILD.gn

@@ -390,7 +390,8 @@ component("skia") {
   "//third_party/skia/src/fonts/SkGScalerContext.h",
   ]
 
-  # need separate win section to handle chromes auto gn filter (build/config/BUILDCONFIG.gn)
+  # need separate win section to handle chromes auto gn filter
+  # (build/config/BUILDCONFIG.gn)
   if (is_win) {
     sources -= [
       "//third_party/skia/src/utils/SkThreadUtils_win.h",
@@ -590,11 +591,15 @@ source_set("skia_opts") {
       "//third_party/skia/include/core/SkTypes.h",
     ]
 
-    # SSE 4
-    if (cpu_arch == "x86") {
-      sources += [ "//third_party/skia/src/opts/SkBlitRow_opts_SSE4_asm.S" ]
-    } else {  # x64
-      sources += [ "//third_party/skia/src/opts/SkBlitRow_opts_SSE4_x64_asm.S" ]
+    if (!is_win) {
+      # SSE 4
+      if (cpu_arch == "x86") {
+        sources += [ "//third_party/skia/src/opts/SkBlitRow_opts_SSE4_asm.S" ]
+      } else {  # x64
+        sources += [
+          "//third_party/skia/src/opts/SkBlitRow_opts_SSE4_x64_asm.S"
+        ]
+      }
     }
 
     if (is_linux || is_mac) {