Enable building angle_perftests on Linux.

BUG=chromium:675997

Change-Id: I3058892b80ccf61664d023419651fd4aa1dfec2a
Reviewed-on: https://chromium-review.googlesource.com/423111
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

src/tests/BUILD.gn

@@ -102,7 +102,7 @@ if (is_win || is_linux || is_mac || is_android) {
   }
 }
 
-if (is_win) {
+if (is_win || is_linux) {
   perftests_gypi = exec_script("//build/gypi_to_gn.py",
                                [
                                  rebase_path("angle_perftests.gypi"),
@@ -120,8 +120,10 @@ if (is_win) {
 
     sources = rebase_path(perftests_gypi.angle_perf_tests_sources, ".", "../..")
 
-    sources +=
-        rebase_path(perftests_gypi.angle_perf_tests_win_sources, ".", "../..")
+    if (is_win) {
+      sources +=
+          rebase_path(perftests_gypi.angle_perf_tests_win_sources, ".", "../..")
+    }
 
     sources += [ "//gpu/angle_perftests_main.cc" ]