infra: Migrate Linux ARM / linux_arm to Goma RBE using ATS

The arm-nacl-clang toolchain is not currently uploaded to Goma RBE.
Instead of adding it (NaCl is deprecated), just use ATS to pass in
the compiler as an input file.

Bug: 950413
Change-Id: I482f8661cf1e1f54468f40cbf6692f48d3d591ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1635116Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Simon Que <sque@chromium.org>
Auto-Submit: Simon Que <sque@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665030}

infra/config/cr-buildbucket.cfg

@@ -2766,6 +2766,7 @@ buckets {
       name: "Linux ARM"
       dimensions: "os:Ubuntu-14.04"
       mixins: "fyi-ci"
+      mixins: "goma-rbe-prod-ats"
     }
     builders {
       name: "Linux"
@@ -3667,6 +3668,7 @@ buckets {
     }
     builders {
       mixins: "linux-try"
+      mixins: "goma-rbe-prod-ats"
       name: "linux_arm"
     }
     builders { mixins: "linux-try" name: "linux_chromium_analysis" }