Warn when unsupported wildcards are used in virtual web tests

Bug: 322720
Change-Id: I04c722cf1443bf09734a1851a4679eafacff22a3
Reviewed-on: https://chromium-review.googlesource.com/1058319
Reviewed-by: Quinten Yearsley <[email protected]>
Commit-Queue: Jeff Carpenter <[email protected]>
Cr-Commit-Position: refs/heads/master@{#558548}
diff --git a/docs/testing/layout_tests.md b/docs/testing/layout_tests.md
index 1673d3b..6d46f348 100644
--- a/docs/testing/layout_tests.md
+++ b/docs/testing/layout_tests.md
@@ -261,6 +261,9 @@
   related to the feature, but they don't scale to flags that make deep
   architectural changes that potentially impact all of the tests.
 
+* Note that using wildcards in virtual test path names (e.g.
+  `virtual/blocking_repaint/fast/repaint/*`) is not supported.
+
 ## Tracking Test Failures
 
 All bugs, associated with layout test failures must have the