Run tests on chromium.clang's ToTAndroid bot

We've had poor Android test coverage on the ToT bots, as shown by the
recent clang roll. Historically that's been due to flakiness of running
Android tests. But now that everything's on swarming it should be
better, and this uses the config from linux_android_rel_ng which runs on
the CQ and seems to be very stable.

Bug: 817298
Change-Id: Ie992c49c9a86037529f8ac6bd70c7ce7b732850c
Reviewed-on: https://chromium-review.googlesource.com/968243Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544022}

testing/buildbot/waterfalls.pyl

@@ -663,6 +663,21 @@
         'additional_compile_targets': [
           'all',
         ],
+        # This mirrors 'KitKat Phone Tester (rel)'.
+        'test_suites': {
+          'gtest_tests': 'chromium_android_gtests',
+          'junit_tests': 'chromium_junit_tests',
+          'isolated_scripts': 'monochrome_apk_checker_isolated_script',
+        },
+        'swarming': {
+          'dimension_sets': [
+            {
+              'device_os': 'KTU84P',
+              'device_type': 'hammerhead',
+            },
+          ],
+        },
+        'os_type': 'android',
       },
       'ToTAndroid x64': {
         'swarming': {