Update linux and mac build docs with jumbo build info

This patch updates the linux and mac build instructions
to mention jumbo builds: how to enable it, and where to
find more information.

Bug: 713137
Change-Id: I95f060660ecdb071b9102c9bba2f45259b08fd77
Reviewed-on: https://chromium-review.googlesource.com/706155
Reviewed-by: Daniel Bratell <bratell@opera.com>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507387}
2 files changed