[Fuchsia] Build Fuchsia runtime on Fuchsia ToT bots

Add checkout_fuchsia to vars in DEPS.
Use the variable to determine whether
to build Fuchsia runtime on ToT bots.

Bug: 1041149
Change-Id: I8f49960e88bce88e50e05f28f7debca800327565
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2774964


Auto-Submit: Chong Gu <chonggu@google.com>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Chong Gu <chonggu@google.com>
Cr-Commit-Position: refs/heads/master@{#864937}