Add finch field trial testing for DecodeWebPToYUV

Add field trial testing for android webview tot the finch experiment
DecodeWebPToYUV.

Bug: 966008
Change-Id: I0939b730e5736b5bc9a6b6a194ee13aa3e8925d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1895513Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Auto-Submit: Sasha McIntosh <sashamcintosh@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712174}

testing/variations/fieldtrial_testing_config.json

@@ -1804,6 +1804,7 @@
         {
             "platforms": [
                 "android",
+                "android_webview",
                 "chromeos",
                 "linux",
                 "mac",