Move cros vm tests to their own swarming pool.

Should help with keeping the vm image warm in the bots' caches.

Currently 20 bots in the pool. Will scale up if needed.

TBR=dpranke@chromium.org

Bug: 840967, 832374
Change-Id: Ia09fdde0a94d0d798cd1c5073f0651b1ec92ac66
Reviewed-on: https://chromium-review.googlesource.com/1052824Reviewed-by: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557303}

testing/buildbot/chromium.fyi.json

@@ -8080,7 +8080,8 @@
           "dimension_sets": [
             {
               "kvm": "1",
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-14.04",
+              "pool": "Chrome-CrOS-VM"
             }
           ]
         },
@@ -8092,7 +8093,8 @@
           "dimension_sets": [
             {
               "kvm": "1",
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-14.04",
+              "pool": "Chrome-CrOS-VM"
             }
           ]
         },
@@ -8104,7 +8106,8 @@
           "dimension_sets": [
             {
               "kvm": "1",
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-14.04",
+              "pool": "Chrome-CrOS-VM"
             }
           ],
           "shards": 2
@@ -8117,7 +8120,8 @@
           "dimension_sets": [
             {
               "kvm": "1",
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-14.04",
+              "pool": "Chrome-CrOS-VM"
             }
           ]
         },
@@ -8129,7 +8133,8 @@
           "dimension_sets": [
             {
               "kvm": "1",
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-14.04",
+              "pool": "Chrome-CrOS-VM"
             }
           ]
         },
@@ -8141,7 +8146,8 @@
           "dimension_sets": [
             {
               "kvm": "1",
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-14.04",
+              "pool": "Chrome-CrOS-VM"
             }
           ]
         },
@@ -8153,7 +8159,8 @@
           "dimension_sets": [
             {
               "kvm": "1",
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-14.04",
+              "pool": "Chrome-CrOS-VM"
             }
           ]
         },
@@ -8176,7 +8183,8 @@
           "dimension_sets": [
             {
               "kvm": "1",
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-14.04",
+              "pool": "Chrome-CrOS-VM"
             }
           ],
           "hard_timeout": 960,

testing/buildbot/waterfalls.pyl

@@ -1414,6 +1414,7 @@
             {
               'kvm': '1',
               'os': 'Ubuntu-14.04',
+              'pool': 'Chrome-CrOS-VM',
             },
           ],
         },