Specify -B <bucket> where to trigger optional tryjobs.

TEST:

$ git cl try -B luci.chromium.try -b linux_upload_clang -b mac_upload_clang -b win_upload_clang -r 001fe5f6
Tried jobs on:
Bucket: luci.chromium.try
  linux_upload_clang: []
  mac_upload_clang: []
  win_upload_clang: []
To see results here, run:        git cl try-results
To see results in browser, run:  git cl web

$ git cl try-results
Scheduled:
  linux_upload_clang    id=8902494487098015248
  mac_upload_clang      id=8902494452269498288
  win_upload_clang      id=8902494452269498272


Bug: 1001522
Change-Id: Ifb37d81b883575e8575f6cdc7cb5f8b8c6a0dda7
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1801923
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696140}

tools/clang/scripts/upload_revision.py

@@ -85,9 +85,11 @@ def main():
       clang_old_revision, clang_svn_revision, commit_message)])
 
   Git(["cl", "upload", "-f", "--bypass-hooks"])
-  Git(["cl", "try", "-b", "linux_upload_clang", "-r", git_revision])
-  Git(["cl", "try", "-b", "mac_upload_clang", "-r", git_revision])
-  Git(["cl", "try", "-b", "win_upload_clang", "-r", git_revision])
+  Git(["cl", "try", "-B", "luci.chromium.try",
+       "-b", "linux_upload_clang",
+       "-b", "mac_upload_clang",
+       "-b", "win_upload_clang",
+       "-r", git_revision])
 
   print ("Please, wait until the try bots succeeded "
          "and then push the binaries to goma.")