Enable prebuild chromium in the build script

see prebuilt chromium commits in build and vendor:

https://github.com/ezio84/android_build/commit/
e5de2b9ba610bacab6b7359714df2bbbada1e48f

https://github.com/ezio84/vendor_slim/commit/
f942f8779edc82335503deea70956b45eadcedee

Change-Id: I99f0bdee126c0b3f7cc2d03eb4a751954aebcfbb