[unified-heap] Add browser flags

Adds flags on all browser layers which are used for enabling in a Finch
trial.

Tbr: jochen@chromium.org
Bug: 843903
Change-Id: I50d9aa479b1fb6e27588eae000138564f377b523
Reviewed-on: https://chromium-review.googlesource.com/c/1286854
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600741}