[gn] Use PIE in v8 executables

This uses the same compiler flag as the chrome executable
does for better performance comparison.

BUG=chromium:474921

Review-Url: https://codereview.chromium.org/2152853004
Cr-Commit-Position: refs/heads/master@{#37798}

gni/v8.gni

@@ -79,6 +79,10 @@ template("v8_executable") {
     configs += invoker.configs
     configs -= remove_configs
     configs += add_configs
+    if (is_linux) {
+      # For enabling ASLR.
+      ldflags = [ "-pie" ]
+    }
   }
 }