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Since https://crrev.com/c/1779543 , `git cl try` now requires the bucket
name to always be specified. This change sets "luci.chromium.try" as the
default bucket when no bucket is configured in builders.json or passed
to `trigger_try_jobs`.

Change-Id: I5d12625f49f3a57c852abdcbefe641b1eb17d7a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1783640
Auto-Submit: Robert Ma <robertma@chromium.org>
Reviewed-by: Luke Z <lpz@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693195}