Add export_compile_commands to root .gn file

With this, gn will emit compile_command.json for all files configured to
be built. This is useful for intellisense in VS Code, for example.

Change-Id: I34a407e7f9445b074d69a14f97753258d0d5a357
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/233355
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
diff --git a/.gn b/.gn
index 40d4d1b..49def81 100644
--- a/.gn
+++ b/.gn
@@ -50,3 +50,7 @@
   use_sysroot = true
   enable_js_protobuf = false
 }
+
+# Export compile_commands.json to build dir for all generated targets.
+# Useful for editors that parse the compilation database for symbol lookup.
+export_compile_commands = [ "*" ]