[Fuchsia] Use AEMU by default on x64 hosts.

Remove the need for --device arg on our FYI and CI bots.

Change-Id: I8115f2d2246b1385d8695926a84a8df51c833214
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2277422Reviewed-by: Robert Ma <robertma@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Chong Gu <chonggu@google.com>
Cr-Commit-Position: refs/heads/master@{#796957}

testing/buildbot/mixins.pyl

@@ -270,13 +270,6 @@
       },
     },
   },
-  'fuchsia_aemu': {
-    '$mixin_append': {
-      'args': [
-        '--device=aemu',
-      ],
-    },
-  },
   'fuchsia_runner_logs': {
     '$mixin_append': {
       'args': [

testing/buildbot/waterfalls.pyl

@@ -2018,7 +2018,6 @@
           'all',
         ],
         'mixins': [
-          'fuchsia_aemu',
           'linux-xenial',
         ],
         'swarming': {
@@ -2039,7 +2038,6 @@
         'browser_config': 'web-engine-shell',
         'os_type': 'linux',
         'mixins': [
-          'fuchsia_aemu',
           'linux-xenial',
         ],
         'swarming': {
@@ -4082,7 +4080,6 @@
         'browser_config': 'web-engine-shell',
         'os_type': 'linux',
         'mixins': [
-          'fuchsia_aemu',
           'linux-xenial',
         ],
         'swarming': {

third_party/blink/tools/blinkpy/web_tests/run_web_tests.py

@@ -142,17 +142,16 @@ def parse_args(args):
             action='store_true',
             default=True,
             help=('Log Zircon debug messages (enabled by default).')),
-        optparse.make_option(
-            '--no-zircon-logging',
-            dest='zircon_logging',
-            action='store_false',
-            default=True,
-            help=('Do not log Zircon debug messages.')),
-        optparse.make_option(
-            '--device',
-            choices=['aemu', 'qemu'],
-            default='qemu',
-            help=('Choose device to launch Fuchsia with.')),
+        optparse.make_option('--no-zircon-logging',
+                             dest='zircon_logging',
+                             action='store_false',
+                             default=True,
+                             help=('Do not log Zircon debug messages.')),
+        optparse.make_option('--device',
+                             choices=['aemu', 'qemu'],
+                             default='aemu',
+                             help=('Choose device to launch Fuchsia with. '
+                                   'Defaults to AEMU.')),
     ]))
 
     option_group_definitions.append((