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This is a reland of bce662eb

It was revert in crrev.com/c/2530204, to make room for other builders.
The host pool looks in a good shape today, so reland this CL
and prepare to fully enable the MM-x86-rel on CQ

Original change's description:
> luci: increase the experimental percentage to 100 from 70 for MM-x86-rel
>
> The pool has plenty of 4-cores for extra builds: http://shortn/_koR1dg8Vvu
> Also the host pool also has plenty of rooms: http://shortn/_xqhW5Z67ft
>
> Bug: 997376, 1127110
> Change-Id: I56b432d960c2f1821934667ac2f38d3fcb4705bf
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2450331
> Reviewed-by: Ben Pastene <bpastene@chromium.org>
> Commit-Queue: Haiyang Pan <hypan@google.com>
> Cr-Commit-Position: refs/heads/master@{#813888}

Bug: 997376
Bug: 1127110
Change-Id: I652429f328af4cb11fe22efc6286af832d77b43b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2535641Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Haiyang Pan <hypan@google.com>
Cr-Commit-Position: refs/heads/master@{#827019}