When would you need this, rather than just downloading all of the new baselines?

Potentially, if you know that you just want to update baselines for a particular builder; then it wouldn't be necessary to wait for all of the try bots to have results before you could download them.

But now I'm inclined to think that this probably unnecessary.

I filed this issue after it was mentioned in chat by zakerinasab@; zakerinasab@, do you think that this feature would be worthwhile?

I think it might get worthy depending on the bug properties. For example, I'm trying to fix a bug related to subscript and superscript in SVG fonts. For some reason the patch works on Linux and Mac (local and try-bots) but when it comes to Windows it works on the local machine but fails on the try-bot (in fact, the behavior is different). In past week I ran rebaseline-cl like 10 times to get the new results (and see the debug info) from try-bots, every time running all android/linux/mac try-bots without needing them.

Ah, I see, so it's important to you to have the debug results locally rather than just clicking through to the build results from the tryserver?

I didn't know that I can download the test results in an archive. So I can either download the results archive and look for my test results in the folder hierarchy to extract for each tryserver, or I can use rebaseline-cl to get the results only for my test. I guess the latter is more convenient and cleaner, but it's not as necessary as I thought.

Yep, once you've run try jobs, the actual results are archived and you can download results for a builder:

e.g. if you
 1. go to a build page: https://build.chromium.org/p/tryserver.blink/builders/mac10.11_blink_rel/builds/1163
 2. click on layout_test_results under archive_webkit_tests_results
 3. click on actual
 4. download, rename, move

then you can get results for an individual builder. "webkit-patch rebaseline-cl" is basically just a tool to download all new baselines for a set of tests at once.

You can download all of the results for a run by clicking on the zip link, too.

Actually, now that I see this again, this is slightly different from  bug 673966 , and Stefan has just uploaded a CL that adds a --builders flag: https://chromium-review.googlesource.com/c/chromium/src/+/724172 :-)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0084b9b88ff3c27f43fff5f9b84c53295ed5f4d3

commit 0084b9b88ff3c27f43fff5f9b84c53295ed5f4d3
Author: Stefan Zager <szager@chromium.org>
Date: Wed Oct 18 00:25:36 2017

Add --builders argument to webkit-patch rebaseline_cl

R=qyearsley@chromium.org

Bug:  658880 
Change-Id: Ifd2ea784a589fba1e34cf2289eba488a25528ebd
Reviewed-on: https://chromium-review.googlesource.com/724172
Commit-Queue: Stefan Zager <szager@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509622}
[modify] https://crrev.com/0084b9b88ff3c27f43fff5f9b84c53295ed5f4d3/third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py
[modify] https://crrev.com/0084b9b88ff3c27f43fff5f9b84c53295ed5f4d3/third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl_unittest.py

After the last change, I'm now getting an error when rebaseline-cl triggers try jobs, fix in https://crrev.com/c/726970

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0646dd9eb594248414bbed2accf646f0fddbb6e6

commit 0646dd9eb594248414bbed2accf646f0fddbb6e6
Author: Quinten Yearsley <qyearsley@chromium.org>
Date: Thu Oct 19 19:28:43 2017

Allow GitCL.trigger_try_jobs to take a frozenset

Bug:  658880 
Change-Id: I8f03010037dbc45daa9165e9e5ae8c47ade889d0
Reviewed-on: https://chromium-review.googlesource.com/726970
Reviewed-by: Stefan Zager <szager@chromium.org>
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510158}
[modify] https://crrev.com/0646dd9eb594248414bbed2accf646f0fddbb6e6/third_party/WebKit/Tools/Scripts/webkitpy/common/net/git_cl.py
[modify] https://crrev.com/0646dd9eb594248414bbed2accf646f0fddbb6e6/third_party/WebKit/Tools/Scripts/webkitpy/common/net/git_cl_unittest.py