gn2bp: Add pre_create_action_module

To support //build:chromeos_buildflags
Add pre_create_action_module for target specific modification before the
create_action_module.
There might be a better way to target specific modification but this
makes //build:chromeos_buildflags specific change small for now.

Test: ./update_result.sh, m cronet_aml_base_base_static
Change-Id: I28e0758f84bebf3405298d7b908b6443914b3944