Put Top Control resizing change behind a flag.

The change to how top controls affect VH units and the initial containing
block was made in r370425. We've decided to hold off on shipping that change
for now so I've placed this change behind a flag. Unless the inert-top-controls
experiment is enabled, the behavior is unchanged from M49.

BUG=428132

Review URL: https://codereview.chromium.org/1721883003

Cr-Commit-Position: refs/heads/master@{#377309}
4 files changed