[infra] Switch back to Xenial on a slow variant

This is to mitigate flaky timeouts due to memory problems on some
Bionic bots.

Bug: v8:11818
Change-Id: I4758f0f167b94d81f43e183a5599a39d8545b4e1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2922245
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Auto-Submit: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74824}

infra/testing/builders.pyl

@@ -352,7 +352,13 @@
       {'name': 'v8testing', 'variant': 'extra', 'shards': 3},
       {'name': 'v8testing', 'variant': 'minor_mc'},
       {'name': 'v8testing', 'variant': 'no_lfa'},
-      {'name': 'v8testing', 'variant': 'stress_instruction_scheduling'},
+      {
+        'name': 'v8testing',
+        'variant': 'stress_instruction_scheduling',
+        'swarming_dimensions' : {
+          'os': 'Ubuntu-16.04',
+        },
+      },
       {
         'name': 'v8testing',
         'variant': 'stress_concurrent_allocation',
@@ -457,7 +463,13 @@
       {'name': 'v8testing', 'variant': 'assert_types'},
       {'name': 'v8testing', 'variant': 'extra'},
       {'name': 'v8testing', 'variant': 'no_lfa'},
-      {'name': 'v8testing', 'variant': 'stress_instruction_scheduling'},
+      {
+        'name': 'v8testing',
+        'variant': 'stress_instruction_scheduling',
+        'swarming_dimensions' : {
+          'os': 'Ubuntu-16.04',
+        },
+      },
     ],
   },
   'v8_linux64_perfetto_dbg_ng_triggered': {
@@ -502,7 +514,13 @@
       {'name': 'v8testing', 'variant': 'minor_mc'},
       {'name': 'v8testing', 'variant': 'no_lfa'},
       {'name': 'v8testing', 'variant': 'slow_path'},
-      {'name': 'v8testing', 'variant': 'stress_instruction_scheduling'},
+      {
+        'name': 'v8testing',
+        'variant': 'stress_instruction_scheduling',
+        'swarming_dimensions' : {
+          'os': 'Ubuntu-16.04',
+        },
+      },
     ],
   },
   'v8_linux64_tsan_rel_ng_triggered': {
@@ -1127,7 +1145,13 @@
       {'name': 'v8testing', 'variant': 'extra'},
       {'name': 'v8testing', 'variant': 'minor_mc'},
       {'name': 'v8testing', 'variant': 'no_lfa'},
-      {'name': 'v8testing', 'variant': 'stress_instruction_scheduling'},
+      {
+        'name': 'v8testing',
+        'variant': 'stress_instruction_scheduling',
+        'swarming_dimensions' : {
+          'os': 'Ubuntu-16.04',
+        },
+      },
       # Noavx.
       {
         'name': 'mozilla',
@@ -1186,7 +1210,13 @@
       {'name': 'v8testing', 'variant': 'minor_mc'},
       {'name': 'v8testing', 'variant': 'no_lfa'},
       {'name': 'v8testing', 'variant': 'slow_path'},
-      {'name': 'v8testing', 'variant': 'stress_instruction_scheduling'},
+      {
+        'name': 'v8testing',
+        'variant': 'stress_instruction_scheduling',
+        'swarming_dimensions' : {
+          'os': 'Ubuntu-16.04',
+        },
+      },
       {
         'name': 'v8testing',
         'variant': 'stress_concurrent_allocation',