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Jumbo is a unity build system for Chromium (see
https://chromium.googlesource.com/chromium/src/+/lkgr/docs/jumbo.md )
which merges cc files at the build target level.

The code in //ppapi needs roughly 15 CPU minutes to compile in my
measurements, which is 2.3% of the total jumbo build time and one of
the largest remaining non-thirdparty code blocks without jumbo support.

This saves about 7.5 CPU minutes in a jumbo build, which is
a bit over 1% of the total reference build time. It doesn't cover
thunk which is most of the remaining compile time. There are
too much generated code with reused symbol names in thunk for it
to be trivially supported right now.

Bug: 898475
Change-Id: I1ee592578ac765cbd58cb856fcd5f331d3dd8c50
Reviewed-on: https://chromium-review.googlesource.com/c/1297366Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#609687}