Add android_webview to the QUIC list of platforms in fieldtrial_testing_config.json

Change-Id: I307c05133fc9188997cd010ce8dd1dcf0657835c
Reviewed-on: https://chromium-review.googlesource.com/1197784
Commit-Queue: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587848}

testing/variations/fieldtrial_testing_config.json

@@ -3450,6 +3450,7 @@
     "QUIC": [
         {
             "platforms": [
+                "android_webview",
                 "android",
                 "chromeos",
                 "ios",