The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/802480762c4dcb5e61bcafc21edf49002ca003f8

commit 802480762c4dcb5e61bcafc21edf49002ca003f8
Author: Quinten Yearsley <qyearsley@google.com>
Date: Fri May 26 17:30:57 2017

rebaseline-cl: If --fill-missing not given, prompt user to fill in missing results.

This would make it so that, for example, if the user runs
"webkit-patch rebaseline-cl" with no arguments and all builders
have results except for android_blink_rel, the tool will print:

There are some builders with no results:
  android_blink_rel
Would you like to try to fill in missing results with
available results? This assumes that layout test results
for the platforms with missing results are the same as
results on other platforms. [y/N]: 

Bug:  724635 
Change-Id: I718b708292be6255561ee97896e6bef8e1f27f43
Reviewed-on: https://chromium-review.googlesource.com/514664
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>
Reviewed-by: Walter Korman <wkorman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#475044}
[modify] https://crrev.com/802480762c4dcb5e61bcafc21edf49002ca003f8/third_party/WebKit/Tools/Scripts/webkitpy/common/system/user_mock.py
[modify] https://crrev.com/802480762c4dcb5e61bcafc21edf49002ca003f8/third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py
[modify] https://crrev.com/802480762c4dcb5e61bcafc21edf49002ca003f8/third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl_unittest.py

Just tested this out on a real change (https://chromium-review.googlesource.com/513527) and the behavior was incorrect:

Would you like to try to fill in missing results with
available results? This assumes that layout test results
for the platforms with missing results are the same as
results on other platforms. [y/N]: y
Aborting.

But after trying again, it worked; It seems like this
might have been because I accidentally added a space before pressing enter. The User.confirm() method isn't very tolerant of accidentally adding spaces. Uploaded https://chromium-review.googlesource.com/517434 to fix this.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/dbb248a23fbf31019a4612268577b0b2371b2aa4

commit dbb248a23fbf31019a4612268577b0b2371b2aa4
Author: Quinten Yearsley <qyearsley@google.com>
Date: Mon May 29 19:56:07 2017

In User.confirm, allow responses like "y " or "yes" to mean yes.

Bug:  724635 
Change-Id: Ib4e6422c69b6c6093b8822cd724d93451efbc884
Reviewed-on: https://chromium-review.googlesource.com/517434
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#475377}
[modify] https://crrev.com/dbb248a23fbf31019a4612268577b0b2371b2aa4/third_party/WebKit/Tools/Scripts/webkitpy/common/system/user.py
[modify] https://crrev.com/dbb248a23fbf31019a4612268577b0b2371b2aa4/third_party/WebKit/Tools/Scripts/webkitpy/common/system/user_unittest.py