Enable arm64 tests on "Fuchsia ARM64" CI builder.

Bug: 1042512
Change-Id: I841d36d7d0a17ae0201f3cb653a1a70ef98b903d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2142325Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Stephen Roe <steveroe@google.com>
Cr-Commit-Position: refs/heads/master@{#757610}

testing/buildbot/waterfalls.pyl

@@ -3863,6 +3863,14 @@
         'additional_compile_targets': [
           'all',
         ],
+        'test_suites': {
+          'gtest_tests': 'fuchsia_gtests',
+        },
+        'mixins': [
+          'arm64',
+          'docker',
+          'linux-xenial',
+        ],
       },
       'Fuchsia x64': {
         'additional_compile_targets': [