Support jumbo in //headless (-12 CPU minutes)

Jumbo is a unity build system for Chromium where many files are
compiled together. Since so much code is in headers and templates
this is much more efficient, and depending on your hardware, can
significantly speed up builds.

The headless module needs 26 CPU minutes to compile without jumbo.
With jumbo for everything, it needs 3.1 CPU minutes. Half of that
is already on master through the jumbo support in mojo. This patch
adds jumbo support for the rest, changing the compilation effort
from 15 CPU minutes to 3.

In real world time, this saves about 1.5 minutes on a full build on an
8 thread machine.

Bug: 883727
Change-Id: I3ede4805064344574c314d03904208110fdcb462
Reviewed-on: https://chromium-review.googlesource.com/1225702
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#593216}
1 file changed