Add target embedding to Safety Tip field trial config.

This CL adds the new target embedding parameter for Safety Tips to the
field trial testing config.  This is a new feature that is only ever
disabled by variations right now, so nothing user-visible is changing.

The parameter was added in crrev.com/c/2119374 .

Change-Id: Ie233c128d814a4048d5b89aca73fb637fda492ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2134951
Auto-Submit: Joe DeBlasio <jdeblasio@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756319}

testing/variations/fieldtrial_testing_config.json

@@ -2186,6 +2186,7 @@
                     "params": {
                         "editdistance": "false",
                         "editdistance_siteengagement": "true",
+                        "targetembedding": "false",
                         "topsites": "true"
                     },
                     "enable_features": [