README: fieldtrial_testing_config.json is not applied to content_browsertests

Source: go/fieldtrial-testing-config#introduction (Google internal)

Bug: 910586
Change-Id: I9bdd750a5712437b952a1220ce5b175dfe4bb21c
Reviewed-on: https://chromium-review.googlesource.com/c/1357541Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613188}

testing/variations/README.md

@@ -10,6 +10,9 @@ perf bots and browser tests in the waterfall.
 > Note: This configuration applies specifically to Chromium developer builds.
 > Chrome branded / official builds do not use these definitions.
 
+> Note: This configuration is NOT used for content_browsertests or other test
+> targets based on content_shell.
+
 ## Config File Format
 
 ```json