[Fuchsia] Enable AEMU on FYI Fuchsia x64 bots.

AEMU was enabled on main waterfall, but not on FYI bots. Enable AEMU on
FYI x64 for consistency.

Bug: 1096611
Change-Id: I337440d94b485af5a8506dc721f0085ed4b6c117
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2268583Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783814}

testing/buildbot/waterfalls.pyl

@@ -1974,6 +1974,7 @@
           'all',
         ],
         'mixins': [
+          'fuchsia_aemu',
           'linux-xenial',
         ],
         'swarming': {
@@ -1994,6 +1995,7 @@
         'browser_config': 'web-engine-shell',
         'os_type': 'linux',
         'mixins': [
+          'fuchsia_aemu',
           'linux-xenial',
         ],
         'swarming': {