testing: use larger vm

This is to fix base_unittests failure in swarming dev builder.
https://luci-milo-dev.appspot.com/p/chromium/builders/ci/linux-rel-swarming

e.g.
https://luci-milo-dev.appspot.com/p/chromium/builders/ci/linux-rel-swarming/6379

I guess 2 core vm does not have sufficient memory for the failed test.
AddressPoolManagerTest.ManyPages

Change-Id: Ie34fb3492cb96f3876b04db89bbd0a272537b7b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2351261Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@google.com>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@google.com>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797202}

testing/buildbot/chrome.ci.json

@@ -3656,7 +3656,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "cores": "2",
+              "cores": "8",
               "os": "Ubuntu-16.04"
             }
           ],
@@ -3787,7 +3787,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "cores": "2",
+              "cores": "8",
               "os": "Ubuntu-16.04"
             }
           ],

testing/buildbot/chromium.dev.json

@@ -642,7 +642,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "cores": "2",
+              "cores": "8",
               "os": "Ubuntu-16.04"
             }
           ],
@@ -773,7 +773,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "cores": "2",
+              "cores": "8",
               "os": "Ubuntu-16.04"
             }
           ],

testing/buildbot/test_suites.pyl

@@ -893,7 +893,7 @@
         'swarming': {
           'dimension_sets': [
             {
-              'cores': '2',
+              'cores': '8',
             },
           ],
         },