In general Google engineers don't tend to work on branches, especially long-running ones. Incremental small code reviews are the expectation. The general process would be to stick things securely behind flags and continue development without turning it on, even if it never ever launches.
Not saying this work should be done -- it shouldn't -- but code being pushed is not the same as "we're going to make this happen tomorrow, no matter what."
My point is that at some other company (e.g. Apple) it would be done in secret on a branch somewhere, then big-bang merged later.
Google's process doesn't tend to work that way.