[Fuchsia] Remove dependency of checkout_aemu variable.

Include AEMU by default in fuchsia package runner scripts for x64 hosts.
Download AEMU package automatically for mac and linux hosts.

Bug: 1000907
Change-Id: Ia0ef798a72ba517616331d8a2dcc9b003b180546
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2026424Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Chong Gu <chonggu@google.com>
Cr-Commit-Position: refs/heads/master@{#737570}

DEPS

@@ -36,7 +36,6 @@ gclient_gn_args = [
   'checkout_nacl',
   'checkout_oculus_sdk',
   'checkout_openxr',
-  'checkout_aemu'
 ]
 
 
@@ -1374,7 +1373,7 @@ deps = {
               'version': 'hvClQbzFhJApEEpHpfc8UqpDdzLK0eabIbmpPlfODp8C'
           },
       ],
-      'condition': 'host_os == "linux" and checkout_aemu',
+      'condition': 'host_os == "linux" and checkout_fuchsia',
       'dep_type': 'cipd',
   },
 
@@ -1385,7 +1384,7 @@ deps = {
               'version': 'T9bWxf8aUC5TwCFgPxpuW29Mfy-7Z9xCfXB9QO8MfU0C'
           },
       ],
-      'condition': 'host_os == "mac" and checkout_aemu',
+      'condition': 'host_os == "mac" and checkout_fuchsia',
       'dep_type': 'cipd',
   },
 

build/config/fuchsia/generate_runner_scripts.gni

@@ -100,7 +100,7 @@ template("fuchsia_package_runner") {
       "${boot_image_root}/qemu/zircon-a.zbi",
     ]
 
-    if (checkout_aemu) {
+    if (host_cpu == "x64" && (host_os == "linux" || host_os == "mac")) {
       data += [ "${aemu_root}/" ]
     }
 

build/fuchsia/aemu_target.py

@@ -25,13 +25,9 @@ class AemuTarget(qemu_target.QemuTarget):
     self._enable_graphics = enable_graphics
     self._hardware_gpu = hardware_gpu
 
-  # TODO(bugs.fuchsia.dev/p/fuchsia/issues/detail?id=37301): Remove
-  # once aemu is part of default fuchsia build
   def _EnsureEmulatorExists(self, path):
     assert os.path.exists(path), \
-          'This checkout is missing %s. To check out the files, add this\n' \
-          'entry to the "custon_vars" section of your .gclient file:\n\n' \
-          '   "checkout_aemu": True\n\n' % (self._emu_type)
+          'This checkout is missing %s.' % (self._emu_type)
 
   def _BuildCommand(self):
     aemu_folder = GetEmuRootForPlatform(self._emu_type)