Show proper message in about Chrome OS page

- Background:
Updates over cellular is blocked by Chrome UI by default, but we now
allow so if user is actively checking for updates (e.g clicking 'check
for updates' button). But auto-updates over cellular is still not
supported.

- Changes in this CL:
Add a |interactive| parameter to IsUpdateOverCellularAllowed(), which
indicates whether user is actively checking for updates.

Show proper warning message when user is over cellular connection.

- Design doc: go/cros-cellular-updates

BUG=chromium:691108
TEST=go/about_chrome_os_err_msg for screenshots of different messages
shown in different situations.
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2897773002
Cr-Commit-Position: refs/heads/master@{#473953}
8 files changed