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This CL adds the checks_only flag to JS Closure Compilations that are only
doing type checking to improve build time.

Before:
5869.41s user 189.50s system 3350% cpu
5861.34s user 190.37s system 3284% cpu

After:
4143.98s user 159.46s system 3020% cpu
4184.40s user 158.95s system 3073% cpu

Bug: 632206
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I68ebdce5a4cf98176d3f55b971e33fc62848d2d4
Reviewed-on: https://chromium-review.googlesource.com/1139840Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575918}