[Benchmarking] pixel2 webview: reduce to 21 shards.

This helps to have backups for when bot temporarily die.
The movement of 7 bots to the weblayer config meant
that until now there were no backups.

Bug: 1022823
Change-Id: I8084caf797e74ac913c1204e09ec3e393a56a3da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1941279
Commit-Queue: Caleb Rouleau <crouleau@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720692}

testing/buildbot/chromium.perf.json

@@ -1359,7 +1359,7 @@
           "hard_timeout": 43200,
           "ignore_task_failure": false,
           "io_timeout": 21600,
-          "shards": 28
+          "shards": 21
         },
         "trigger_script": {
           "args": [

tools/perf/core/bot_platforms.py

@@ -268,7 +268,7 @@ ANDROID_PIXEL2 = PerfPlatform(
     _ANDROID_PIXEL2_BENCHMARK_CONFIGS, 35, 'android')
 ANDROID_PIXEL2_WEBVIEW = PerfPlatform(
     'android-pixel2_webview-perf', 'Android OPM1.171019.021',
-    _ANDROID_PIXEL2_WEBVIEW_BENCHMARK_CONFIGS, 28, 'android')
+    _ANDROID_PIXEL2_WEBVIEW_BENCHMARK_CONFIGS, 21, 'android')
 ANDROID_PIXEL2_WEBLAYER = PerfPlatform(
     'android-pixel2_weblayer-perf', 'Android OPM1.171019.021',
     _ANDROID_PIXEL2_WEBLAYER_BENCHMARK_CONFIGS, 4, 'android')