[Perf] Change android-pixel2-perf to 28 shards

This bot frequently fails due to insufficient capacity. Reducing
number of shards from 35 to 28.

Bug: 1114129
Change-Id: Id7a4fdb96725b815db009cd77bb01a4bf61c2c3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2364515
Commit-Queue: John Chen <johnchen@chromium.org>
Commit-Queue: Wenbin Zhang <wenbinzhang@google.com>
Auto-Submit: John Chen <johnchen@chromium.org>
Reviewed-by: Wenbin Zhang <wenbinzhang@google.com>
Cr-Commit-Position: refs/heads/master@{#799708}

testing/buildbot/chromium.perf.json

@@ -523,7 +523,7 @@
           "ignore_task_failure": false,
           "io_timeout": 21600,
           "service_account": "chrome-tester@chops-service-accounts.iam.gserviceaccount.com",
-          "shards": 35
+          "shards": 28
         },
         "trigger_script": {
           "args": [

tools/perf/core/bot_platforms.py

@@ -469,7 +469,7 @@ ANDROID_NEXUS_5X_WEBVIEW = PerfPlatform(
 ANDROID_PIXEL2 = PerfPlatform('android-pixel2-perf',
                               'Android OPM1.171019.021',
                               _ANDROID_PIXEL2_BENCHMARK_CONFIGS,
-                              35,
+                              28,
                               'android',
                               executables=_ANDROID_PIXEL2_EXECUTABLE_CONFIGS,
                               run_reference_build=False)