Move linux-chromeos-chrome's browser_tests into its own swarming pool.

To prevent it from leaving junk around on the machine that causes
other tests in that same pool to fail.

Bug: 1039746
Change-Id: I58a9bc242c56f5d7af196335cdd7d621cdb90a15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1995779Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730595}

testing/buildbot/chrome.json

@@ -1103,7 +1103,7 @@
           "dimension_sets": [
             {
               "os": "Ubuntu-16.04",
-              "pool": "chrome.tests",
+              "pool": "chrome.tests.browser_tests_killer",
               "ssd": "0"
             }
           ],

testing/buildbot/test_suite_exceptions.pyl

@@ -167,6 +167,14 @@
         'args': [
           '--test-launcher-filter-file=../../testing/buildbot/filters/chromeos.browser_tests.filter',
         ],
+        # TODO(https://crbug.com/1039746): Move this back to the main pool.
+        'swarming': {
+          'dimension_sets': [
+            {
+              'pool': 'chrome.tests.browser_tests_killer',
+            },
+          ],
+        },
       },
       'linux-chromeos-code-coverage': {
         'swarming': {