To speed up compilation times, jumbo allows files to be compiled
together. This is a well known method ("unity builds") to both
compile faster and create a poor man's "full program optimization".
We are only interested in compile times.

Background:
https://chromium.googlesource.com/chromium/src/+/master/docs/jumbo.md

Note that jumbo builds are not enabled by default.  To try this out,
add use_jumbo_build=true to your GN args.

BUG=chromium:746958

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ieb9fdccb6c135e9806dbed91c09a29aa8b8bee11
Reviewed-on: https://chromium-review.googlesource.com/579090
Commit-Queue: Mostyn Bramley-Moore <mostynb@opera.com>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47239}