Add android_webview to SetLowPriorityForBeacon field trial config

This is needed to start the experiment on beta.

Bug: 1018081
Change-Id: I46aa390c69c9b152d0a217618cfc0b6bb46f4c37
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906856
Auto-Submit: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714286}

testing/variations/fieldtrial_testing_config.json

@@ -5753,6 +5753,7 @@
         {
             "platforms": [
                 "android",
+                "android_webview",
                 "chromeos",
                 "linux",
                 "mac",