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This adds a gn arg, use_goma_thin_lto. When true, this enables a
linker wrapper which causes ThinLTO code generation to be distributed
using Goma. This can be used to reduce build time compared to a
non-distributed ThinLTO build. The new gn arg allows this feature
to be controlled by the build configuration, allowing distributed
ThinLTO to be rolled out gradually. The arg defaults to false.

R=akhuang,gbiv,hans,rnk,thakis

Bug: 877722
Change-Id: Ibe4dac1669b297e8cb865dcfd3fe512f8f0cdf60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2067858
Commit-Queue: Bob Haarman <inglorion@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744473}