Update device_os dimension for webview network test bot.

The phones were flashed to aosp recently and the exact version changed
a little.

R=jbudorick
NOTRY=true  # crbug.com/906840 for akar failures

Bug: 880623
Change-Id: If0e0b4f8e5cf5bcc6269b879af873f0f8dc5bf84
Reviewed-on: https://chromium-review.googlesource.com/c/1343295
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609529}

testing/buildbot/chromium.android.fyi.json

@@ -694,7 +694,7 @@
           ],
           "dimension_sets": [
             {
-              "device_os": "OPM1.171019.021",
+              "device_os": "OPD3.170816.023",
               "device_type": "walleye",
               "os": "Android"
             }

testing/buildbot/waterfalls.pyl

@@ -574,7 +574,7 @@
         'swarming': {
           'dimension_sets': [
             {
-              'device_os': 'OPM1.171019.021',
+              'device_os': 'OPD3.170816.023',
               'device_type': 'walleye',
               'os': 'Android',
             },