Adding SwiftShader compilation to Linux

TBR=thakis@chromium.org
BUG=630728

Review-Url: https://codereview.chromium.org/2514823002
Cr-Commit-Position: refs/heads/master@{#433640}
diff --git a/BUILD.gn b/BUILD.gn
index 4eb68fc..2f894ad6 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -525,7 +525,7 @@
     ]
   }
 
-  if (is_win || is_mac) {
+  if (is_win || is_mac || is_linux) {
     deps += [ "//third_party/swiftshader" ]
   }