[web] increase number of shards. sync engine web tests same as flutter repo

.cirrus.yml

@@ -4,9 +4,10 @@ web_shard_template: &WEB_SHARD_TEMPLATE
   only_if: "changesInclude('.cirrus.yml', 'DEPS', 'lib/web_ui/**', 'web_sdk/**') || $CIRRUS_PR == ''"
   environment:
     # As of March 2020, the Web shards needed 16G of RAM and 4 CPUs to run all framework tests with goldens without flaking.
+    WEB_SHARD_COUNT: 12
     CPU: 4
     MEMORY: 16G
-    WEB_SHARD_COUNT: 4
+    CHROME_NO_SANDBOX: true
   compile_host_script: |
     cd $ENGINE_PATH/src
     ./flutter/tools/gn --unoptimized --full-dart-sdk
@@ -108,7 +109,31 @@ task:
     - name: web_tests-2-linux
       << : *WEB_SHARD_TEMPLATE
 
-    - name: web_tests-3_last-linux # last Web shard must end with _last
+    - name: web_tests-3-linux
+      << : *WEB_SHARD_TEMPLATE
+
+    - name: web_tests-4-linux
+      << : *WEB_SHARD_TEMPLATE
+
+    - name: web_tests-5-linux
+      << : *WEB_SHARD_TEMPLATE
+
+    - name: web_tests-6-linux
+      << : *WEB_SHARD_TEMPLATE
+
+    - name: web_tests-7-linux
+      << : *WEB_SHARD_TEMPLATE
+
+    - name: web_tests-8-linux
+      << : *WEB_SHARD_TEMPLATE
+
+    - name: web_tests-9-linux
+      << : *WEB_SHARD_TEMPLATE
+
+    - name: web_tests-10-linux
+      << : *WEB_SHARD_TEMPLATE
+
+    - name: web_tests-11_last-linux # last Web shard must end with _last
       << : *WEB_SHARD_TEMPLATE
 
     - name: build_test