Enable prebuild chromium in the build script

see prebuilt chromium commits in build and vendor:

https://github.com/ezio84/android_build/commit/
b249d58c68fcba09823e24d4667dee3cf40407bb

https://github.com/ezio84/vendor_slim/commit/
fb0ea774133c4407952761decbd967f0b3e49fca

Change-Id: I99f0bdee126c0b3f7cc2d03eb4a751954aebcfbb