Enable prebuild chromium in the build script

see prebuilt chromium commits in build and vendor:

https://github.com/ezio84/android_build/commit/
9e67e44a406b930847ab0d04fdd07abfe654e0e4

https://github.com/ezio84/vendor_slim/commit/
5fa1c412bb160cb9b3d8917b7bba35a37d5e9896

Change-Id: I99f0bdee126c0b3f7cc2d03eb4a751954aebcfbb