Add shared highlights to testing config

Bug: None
Change-Id: I07f79c46658bb206a084a21a09e7b2c1b9308a1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532724Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Gayane Petrosyan <gayane@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826792}

testing/variations/fieldtrial_testing_config.json

@@ -1546,6 +1546,7 @@
                 {
                     "name": "Enabled",
                     "enable_features": [
+                        "ChromeShareHighlightsAndroid",
                         "ChromeShareQRCode",
                         "ChromeShareScreenshot",
                         "ChromeSharingHub",