[fuchsia] Enable fuchsia system log collection on fuchsia-fyi-arm64-rel waterfall.

Logs are being collected on Arm64 FYI to check a problem with viz_unittests on Arm64 CQ dry runs before enabling log collection on CI.

Bug: 1042512
Change-Id: Id92e3285fb826001a62c8d0a0c548c56411999b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2149278Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Stephen Roe <steveroe@google.com>
Cr-Commit-Position: refs/heads/master@{#758943}

testing/buildbot/mixins.pyl

@@ -234,6 +234,13 @@
       ],
     },
   },
+  'fuchsia_system_log': {
+    '$mixin_append': {
+      'args': [
+        '--enable-fuchsia-system-log',
+      ],
+    },
+  },
   'gpu_pool': {
     'swarming': {
       'dimensions': {

testing/buildbot/waterfalls.pyl

@@ -1864,6 +1864,7 @@
         'mixins': [
           'arm64',
           'docker',
+          'fuchsia_system_log',
           'linux-xenial',
         ],
       },
@@ -1872,6 +1873,7 @@
           'all',
         ],
         'mixins': [
+          'fuchsia_system_log',
           'linux-xenial',
         ],
         'swarming': {
@@ -1892,6 +1894,7 @@
         'browser_config': 'web-engine-shell',
         'os_type': 'linux',
         'mixins': [
+          'fuchsia_system_log',
           'linux-xenial',
         ],
         'swarming': {