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This requires goma client 193. Without it, this will cause every compile
job to fall back to local. It was released ~7pm Eastern on Mar 4 2020.

Goma should auto-update. You can see your current version in
`cat ~/goma/MANIFEST` / `type c:\src\goma\goma-win64\MANIFEST`
(the VERSION= line).

If it's out-of-date, you can explicitly update with
`~/goma/goma_ctl.py update` / `c:\src\goma\goma-win64\goma_ctl.bat update`

This flag omits system includes from /showIncludes output. This matches
what we do on non-Win (-MMD instead of -MD). We already make sure
we compile after system header updates (win sdk, libc++, compiler
builtin headers) due to changing the compile commandline after updates,
so there's no drawback to using this flag. It reduces the size of
.ninja_deps after a full build from 126MB to 49MB, reduces incremental
build time by 0.8s on Linux in a cross build (according to faster deps
load and fewer stats and a lower average stat time according to
-d states, probably because the SDK headers are in a fuse mount with
relatively slow stat()ing), and reduces the amount of data that ninja
needs to parse out of the compiler during a build of 'chrome' from
1.3GG to 627MB (according to `ninja -t deps`). Since that's on the
critical job scheduling path, it probably helps goma build speed too.

The flag was added in https://reviews.llvm.org/D75093

Bug: none
Change-Id: I7afe7ce2260b86993ba0cef9cb5a3da0b6803aa7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078738
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747672}