Fix webview max restarts when --device-serial is not specified.

Test: wpt run --test-type=testharness android_webview --no-manifest-download
Change-Id: Iefbfb67d37be0281c0520fb9a6cb7e23995c611b
Bug: 1011098
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1867038
Commit-Queue: Andrew Luo <aluo@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707158}

third_party/blink/web_tests/external/wpt/tools/wptrunner/wptrunner/browsers/android_webview.py

@@ -54,7 +54,7 @@ def executor_kwargs(test_type, server_config, cache_manager, run_info_data,
     capabilities["goog:chromeOptions"]["androidPackage"] = \
         "org.chromium.webview_shell"
     capabilities["goog:chromeOptions"]["androidActivity"] = ".WebPlatformTestsActivity"
-    if 'device_serial' in kwargs:
+    if kwargs.get('device_serial'):
         capabilities["goog:chromeOptions"]["androidDeviceSerial"] = kwargs['device_serial']
 
     # Workaround: driver.quit() cannot quit SystemWebViewShell.