emulator: Have the test suites on Pie builder running on 4-core GCE

chrome_public_test_apk still runs on 8-core, to shorten test runtime.
Also increase the shards to 20 for content_browsertests to avoid timeout.

Bug: 922145
Change-Id: I82cf32c7afd7403f0ad7243ed29afdbb9688fa4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2332444Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Haiyang Pan <hypan@google.com>
Cr-Commit-Position: refs/heads/master@{#814938}

testing/buildbot/mixins.pyl

@@ -251,16 +251,6 @@
       },
     },
   },
-  'emulator-8-cores': {
-    'swarming': {
-      'dimensions': {
-        'device_os': None,
-        'device_type': None,
-        'pool': 'chromium.tests.avd',
-        'machine_type': 'n1-standard-8',
-      },
-    },
-  },
   'flounder': {
     # Nexus 9
     'swarming': {

testing/buildbot/test_suite_exceptions.pyl

@@ -771,7 +771,12 @@
               'name': 'system_images_android_28_google_apis_playstore_x86',
               'path': '.emulator_sdk',
             },
-          ]
+          ],
+          'dimension_sets': [
+            {
+              'machine_type': 'n1-standard-8', # use 8-core to shorten runtime
+            },
+          ],
         },
       },
     },
@@ -981,7 +986,7 @@
           '--test-launcher-filter-file=../../testing/buildbot/filters/android.emulator_p.content_browsertests.filter',
         ],
         'swarming': {
-          'shards': 15,
+          'shards': 20,
         },
       },
       'linux-blink-heap-concurrent-marking-tsan-rel': {
@@ -2001,7 +2006,12 @@
               'name': 'system_images_android_28_google_apis_playstore_x86',
               'path': '.emulator_sdk',
             },
-          ]
+          ],
+          'dimension_sets': [
+            {
+              'machine_type': 'n1-standard-8', # use 8-core to shorten runtime
+            },
+          ],
         },
       },
     },
@@ -2058,7 +2068,7 @@
           '--test-launcher-filter-file=../../testing/buildbot/filters/android.emulator_p.content_browsertests.filter',
         ],
         'swarming': {
-          'shards': 15,
+          'shards': 20,
         },
       },
     },

testing/buildbot/waterfalls.pyl

@@ -1028,7 +1028,7 @@
       'android-pie-x86-rel': {
         'mixins': [
           'pie-x86-emulator',
-          'emulator-8-cores',
+          'emulator-4-cores',
           'linux-xenial',
           'x86-64',
         ],