bind-gen: Add a flag to use the new IDL interface implementation

This patch allows the following branch.
    #if defined(USE_BLINK_V8_BINDING_NEW_IDL_INTERFACE)
    #else
    #endif

Bug: 839389
Change-Id: I17249cf1a9c84bf11a52f31cfad41b3c1378431f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2275287Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784412}

third_party/blink/renderer/config.gni

@@ -39,6 +39,10 @@ declare_args() {
 
   # If true, webgl2-compute context will be supported.
   support_webgl2_compute_context = !is_android
+
+  # If true, the new implementation (experimental) of Blink-V8 bindings
+  # (of IDL interface) is used.
+  use_blink_v8_binding_new_idl_interface = false
 }
 
 # feature_defines_list ---------------------------------------------------------
@@ -61,6 +65,10 @@ if (use_webaudio_pffft) {
   feature_defines_list += [ "WTF_USE_WEBAUDIO_PFFFT=1" ]
 }
 
+if (use_blink_v8_binding_new_idl_interface) {
+  feature_defines_list += [ "USE_BLINK_V8_BINDING_NEW_IDL_INTERFACE" ]
+}
+
 if (blink_symbol_level == 2) {
   blink_symbols_config = [ "//build/config/compiler:symbols" ]
 } else if (blink_symbol_level == 1) {