Sets uncompiled_source web_request_internal for schema on GN builds.

BUG=None
R=rockot@chromium.org
TEST=None

Review URL: https://codereview.chromium.org/642353003

Cr-Commit-Position: refs/heads/master@{#299183}

extensions/common/api/schemas.gni

@@ -10,4 +10,6 @@ gypi_values = exec_script(
 
 sources = gypi_values.main_schema_files
 
+uncompiled_sources = gypi_values.main_non_compiled_schema_files
+
 root_namespace = "extensions::core_api::%(namespace)s"