Use swarming on ToTLinuxThinLTO.

The lack of swarming was the most likely cause of mysterious test
failures that weren't affecting any other builder.

Bug: 849868
Change-Id: I6ee8b78886dce01a319062b3d6526381bb25ecf3
Reviewed-on: https://chromium-review.googlesource.com/1087818Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564737}

testing/buildbot/waterfalls.pyl

@@ -652,7 +652,6 @@
         'test_suites': {
           'gtest_tests': 'chromium_linux_clang_and_gl_gtests',
         },
-        'use_swarming': False,
       },
       'ToTLinuxUBSanVptr': {
         'additional_compile_targets': [