Require kvm for browser tests

Crostini tests were failing on E2 machines because there was no KVM available.
This CL requires that browser tests are run on machines that have a KVM.

Bug: 1058924
Change-Id: I9178dd5c8fb8e704e853c75c58cf5d0899c4c593
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088408
Commit-Queue: Stephanie Kim <kimstephanie@google.com>
Reviewed-by: Aaron Gable <agable@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747454}

testing/buildbot/chromium.chromiumos.json

@@ -2539,6 +2539,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
+              "kvm": "1",
               "os": "Ubuntu-16.04"
             }
           ],

testing/buildbot/test_suite_exceptions.pyl

@@ -522,6 +522,11 @@
       'linux-chromeos-rel': {
         'swarming': {
           'shards': 20,
+          'dimension_sets': [
+            {
+              'kvm': '1',
+            },
+          ],
         },
       },
       'linux-code-coverage': {