Add system_webview_apk back to gn_isolate_map.

I removed it in https://chromium-review.googlesource.com/c/chromium/src/+/595248.

Bug: 769899
Change-Id: I84eb38e697ee4f66329c6098b5431c1907b53d4e
Reviewed-on: https://chromium-review.googlesource.com/691277Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Benjamin Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505258}

testing/buildbot/gn_isolate_map.pyl

@@ -888,6 +888,10 @@
     "label": "//chrome/test:sync_integration_tests",
     "type": "windowed_test_launcher",
   },
+  "system_webview_apk": {
+    "label": "//android_webview:system_webview_apk",
+    "type": "additional_compile_target",
+  },
   "system_webview_shell_layout_test_apk": {
     "label": "//android_webview/tools/system_webview_shell:system_webview_shell_layout_test_apk",
     "type": "console_test_launcher",

testing/buildbot/manage.py

@@ -130,6 +130,7 @@ SKIP_GN_ISOLATE_MAP_TARGETS = {
   'net_junit_tests',
   'net_junit_tests',
   'service_junit_tests',
+  'system_webview_apk',
   'ui_junit_tests',
   'vrcore_fps_test',
   'vr_perf_tests',