Prepare for |is_posix| switch in the Fuchsia build

|is_posix| will be switched to false for Fuchsia, this is a preliminary change.

Bug: chromium:812974
Change-Id: I7028fe83d05a46af7186fe5b1b963016fd35f334
Reviewed-on: https://webrtc-review.googlesource.com/65781
Reviewed-by: Raymond Toy <rtoy@google.com>

dl/BUILD.gn

diff --git a/dl/BUILD.gn b/dl/BUILD.gn
index 76c3a3f..a3e1dd3 100644
--- a/dl/BUILD.gn
+++ b/dl/BUILD.gn
@@ -116,7 +116,7 @@
   }
 
   if (current_cpu == "x86" || current_cpu == "x64") {
-    if (is_posix) {
+    if (is_posix || is_fuchsia) {
       cflags += [ "-msse2" ]
     }