Make CrOS VM tests of url_unittests prefer bots they've already run on.

This moves that particular test to the main chromium.tests pool while
also configuring it to use a named cache on the test bot. This
cache won't be used to store any files, but will simply let us target
(via swarming dimensions) bots that already have this cache (i.e. bots
that have already ran a CrOS VM url_unittests).

This is done by also adding an additional task slice to the test that
will wait 60s for a bot with that cache already present. If one isn't
available in time, it will run on any bot. In the latter case, that
bot will then have our magic cache created on it, allowing it to run
future CrOS VM url_unittests.

Bug: 1132206
Change-Id: If8630a84bb9892065deb8e3639e3d0780e2ffc83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438776Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Haiyang Pan <hypan@google.com>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814279}

testing/buildbot/chromium.chromiumos.json

@@ -596,11 +596,25 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
+              "cpu": "x86",
               "kvm": "1",
               "os": "Ubuntu-16.04",
-              "pool": "chromium.tests.cros.vm"
+              "pool": "chromium.tests"
+            }
+          ],
+          "named_caches": [
+            {
+              "name": "cros_vm",
+              "path": "magic_cros_vm_cache"
             }
           ],
+          "optional_dimensions": {
+            "60": [
+              {
+                "caches": "cros_vm"
+              }
+            ]
+          },
           "service_account": "chromium-tester@chops-service-accounts.iam.gserviceaccount.com"
         },
         "test": "url_unittests",

testing/buildbot/test_suite_exceptions.pyl

@@ -2705,6 +2705,33 @@
     },
   },
   'url_unittests': {
+    'modifications': {
+      # TODO(crbug.com/1132206): Apply this to all of the bot's tests if the
+      # cache affinity works.
+      'chromeos-amd64-generic-rel': {
+        'swarming': {
+          'dimension_sets': [
+            {
+              'cpu': 'x86',
+              'pool': 'chromium.tests',
+            }
+          ],
+          'optional_dimensions': {
+            '60': [
+              {
+                'caches': 'cros_vm',
+              }
+            ],
+          },
+          'named_caches': [
+            {
+              'name': 'cros_vm',
+              'path': 'magic_cros_vm_cache',
+            },
+          ],
+        },
+      },
+    },
     'remove_from': [
       # crbug.com/1054240
       'Fuchsia ARM64',