[wpt import] allow importing the docs/ directory from upstream.

In PR https://github.com/web-platform-tests/wpt/pull/23922 a command was
added to the docs/ directory, and docs/ was added to tools/wpt/paths.
This means that running wpt now requires docs/command.json to exist.

The WPT Waterfall (go/wpt-waterfall) is currently broken by this.

Change-Id: Ie5514a6257f1d87f7a3da1fa758e655dd2312841
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2229063
Commit-Queue: Luke Z <lpz@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Auto-Submit: Luke Z <lpz@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774666}

third_party/blink/web_tests/W3CImportExpectations

@@ -109,7 +109,6 @@ external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/transforms [ Sk
 external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/ui3 [ Skip ]
 external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/values3 [ Skip ]
 external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/will-change [ Skip ]
-external/wpt/docs [ Skip ]
 external/wpt/dpub-aam [ Skip ]
 external/wpt/dpub-aria [ Skip ]
 external/wpt/html-longdesc [ Skip ]