[Benchmarking] Use WebLayerShellSystemWebView.apk implicitly.

https://chromium-review.googlesource.com/c/catapult/+/1961011 makes it so that
if you do no specify a --webview-embedder-apk flag then we will use
WebLayerShellSystemWebView.apk for a weblayer browser.

It is also nicer not to explicit specify a flag for the reasons described
in
https://chromium.googlesource.com/chromium/src.git/+/c1324fa4ee0802370abbd624df217ef7b479180c

R=johnchen@chromium.org

Bug: 1028125
Change-Id: I01ac075f8ef92928ca9e15dbb665ea9ce8c95f77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1981068Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727484}

testing/buildbot/chromium.perf.json

@@ -1283,9 +1283,7 @@
           "-v",
           "--browser=android-weblayer",
           "--upload-results",
-          "--test-shard-map-filename=android-pixel2_weblayer-perf_map.json",
-          "--webview-embedder-apk=../../out/Release/apks/WebLayerShell.apk",
-          "--webview-embedder-apk=../../out/Release/apks/WebLayerSupport.apk"
+          "--test-shard-map-filename=android-pixel2_weblayer-perf_map.json"
         ],
         "isolate_name": "performance_weblayer_test_suite",
         "merge": {

tools/perf/core/perf_data_generator.py

@@ -1174,12 +1174,6 @@ def generate_telemetry_args(tester_config, platform):
                      tester_config['browser'])
     if tester_config['platform'].startswith('android'):
       test_args.append('--device=android')
-
-  if tester_config['platform'] == 'android-weblayer':
-    test_args.append(
-        '--webview-embedder-apk=../../out/Release/apks/WebLayerShell.apk')
-    test_args.append(
-        '--webview-embedder-apk=../../out/Release/apks/WebLayerSupport.apk')
   return test_args