Two recent examples of this on the tryservers:

https://ci.chromium.org/p/chromium/builders/luci.chromium.try/win10_chromium_x64_rel_ng/84677
https://chromium-swarm.appspot.com/task?id=3fa439f416126e10&refresh=10

https://ci.chromium.org/p/chromium/builders/luci.chromium.try/win10_chromium_x64_rel_ng/84581
https://chromium-swarm.appspot.com/task?id=3fa3c8cc3017ae10&refresh=10&show_raw=1

In both cases the symptom is the same; an unkillable renderer process was left running, causing Swarming to fail the entire shard. Log excerpt from the first:

-----
Retrying 1 test (retry #1)
[948/948] SpellcheckServiceBrowserTest.MultilingualPreferenceNotMigrated (2183 ms)
SUCCESS: all tests passed.
Failed to delete C:\b\s\w\ir (12 files remaining).
  Maybe the test has a subprocess outliving it.
  Sleeping 2 seconds.
Failed to delete C:\b\s\w\ir (12 files remaining).
  Maybe the test has a subprocess outliving it.
  Sleeping 4 seconds.
Failed to delete C:\b\s\w\ir. The following files remain:
- \\?\C:\b\s\w\ir
- \\?\C:\b\s\w\ir\out
- \\?\C:\b\s\w\ir\out\Release_x64
- \\?\C:\b\s\w\ir\out\Release_x64\browser_tests.exe
- \\?\C:\b\s\w\ir\out\Release_x64\chrome_100_percent.pak
- \\?\C:\b\s\w\ir\out\Release_x64\chrome_200_percent.pak
- \\?\C:\b\s\w\ir\out\Release_x64\icudtl.dat
- \\?\C:\b\s\w\ir\out\Release_x64\locales
- \\?\C:\b\s\w\ir\out\Release_x64\locales\en-US.pak
- \\?\C:\b\s\w\ir\out\Release_x64\natives_blob.bin
- \\?\C:\b\s\w\ir\out\Release_x64\resources.pak
- \\?\C:\b\s\w\ir\out\Release_x64\v8_context_snapshot.bin
Enumerating processes:
- pid 8656; Handles: 274; Exe: C:\b\s\w\ir\out\Release_x64\browser_tests.exe; Cmd: "C:\b\s\w\ir\out\Release_x64\browser_tests.exe" --type=renderer --disable-compositor-ukm-for-tests --dom-automation --enable-logging=stderr --file-url-path-alias="/gen=C:\b\s\w\ir\out\Release_x64\gen" --force-color-profile=srgb --ipc-connection-timeout=30 --test-type=browser --field-trial-handle=1468,3678886453054102039,4181609478204832977,131072 --enable-features=TestFeatureForBrowserTest1 --disable-features=NetworkPrediction,SpeculativePreconnect,TestFeatureForBrowserTest2,WebRTC-H264WithOpenH264FFmpeg --service-pipe-token=8227455578678677143 --lang=en-US --noerrdialogs --user-data-dir="C:\b\s\w\ittrto8m\scoped_dir8664_3332\d8664_9461" --enable-offline-auto-reload-visible-only --start-stack-profiler --device-scale-factor=1 --num-raster-threads=4 --enable-main-frame-before-activation --service-request-channel-token=8227455578678677143 --renderer-client-id=3 --no-v8-untrusted-code-mitigations --mojo-platform-channel-handle=2332 /prefetch:1
Terminating 1 processes:
- failed to kill 8656
...
1364 2018-08-30 15:36:48.115 E: Failure with [Error 5] Access is denied: u'\\\\?\\C:\\b\\s\\w\\ir\\out\\Release_x64\\browser_tests.exe'
*** Swarming tried multiple times to delete the run directory and failed ***
*** Hard failing the task ***

Swarming detected that your testing script ran an executable, which may have
started a child executable, and the main script returned early, leaving the
children executables playing around unguided.

You don't want to leave children processes outliving the task on the Swarming
bot, do you? The Swarming bot doesn't.

How to fix?
- For any process that starts children processes, make sure all children
  processes terminated properly before each parent process exits. This is
  especially important in very deep process trees.
  - This must be done properly both in normal successful task and in case of
    task failure. Cleanup is very important.
- The Swarming bot sends a SIGTERM in case of timeout.
  - You have 30.0 seconds to comply after the signal was sent to the process
    before the process is forcibly killed.
- To achieve not leaking children processes in case of signals on timeout, you
  MUST handle signals in each executable / python script and propagate them to
  children processes.
  - When your test script (python or binary) receives a signal like SIGTERM or
    CTRL_BREAK_EVENT on Windows), send it to all children processes and wait for
    them to terminate before quitting.

See
https://chromium.googlesource.com/infra/luci/luci-py.git/+/master/appengine/swarming/doc/Bot.md#Graceful-termination_aka-the-SIGTERM-and-SIGKILL-dance
for more information.
-----

Not sure how long this has been happening or whether this is a recently introduced problem. Only seen on Windows so far.