Enable TextFragmentAnchor finch experiment for webview

Fixes the field trial config as required for cl/285264679

Bug: 978139
Change-Id: Iade5a7f945d8363ca274a0f61cfe34e5a0bb0e79
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1993942
Auto-Submit: Nick Burris <nburris@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730232}

testing/variations/fieldtrial_testing_config.json

@@ -6629,6 +6629,7 @@
         {
             "platforms": [
                 "android",
+                "android_webview",
                 "chromeos",
                 "linux",
                 "mac",