[inspector] expose js_protocol.json file location in BUILD file

This location is used at least in Chromium.

BUG=chromium:635948
R=dgozman@chromium.org

Review-Url: https://codereview.chromium.org/2405303002
Cr-Commit-Position: refs/heads/master@{#40225}

gni/v8.gni

@@ -38,15 +38,17 @@ if (v8_enable_backtrace == "") {
   v8_enable_backtrace = is_debug && !v8_optimized_debug
 }
 
-###############################################################################
-# Templates
-#
-
 # Points to // in v8 stand-alone or to //v8/ in chromium. We need absolute
 # paths for all configs in templates as they are shared in different
 # subdirectories.
 v8_path_prefix = get_path_info("../", "abspath")
 
+v8_inspector_js_protocol = v8_path_prefix + "/src/inspector/js_protocol.json"
+
+###############################################################################
+# Templates
+#
+
 # Common configs to remove or add in all v8 targets.
 v8_remove_configs = [ "//build/config/compiler:chromium_code" ]
 v8_add_configs = [