Convert ENABLE_WAYLAND_SERVER to a build flag.

This way, ENABLE_WAYLAND_SERVER isn't defined globally, but only where
it's needed (chrome/common and deps). No intended behavior change.

BUG=none

Review-Url: https://codereview.chromium.org/2741783007
Cr-Commit-Position: refs/heads/master@{#456293}
7 files changed