Enable optimization when compiling for NaCl with GN

PNaCl code validation fails when compiling with -O0 due to the linked
bug. Always enable optimization when compiling for NaCl to workaround
this bug.

BUG=538243
R=fbarchard@chromium.org

Review URL: https://codereview.chromium.org/1403813002 .

BUILD.gn

@@ -102,6 +102,12 @@ source_set("libyuv") {
   if (use_neon) {
     deps += [ ":libyuv_neon" ]
   }
+
+  if (is_nacl) {
+    # Always enable optimization under NaCl to workaround crbug.com/538243 .
+    configs -= [ "//build/config/compiler:default_optimization" ]
+    configs += [ "//build/config/compiler:optimize_max" ]
+  }
 }
 
 if (use_neon) {