Introduce an ObserverList<T>::Unchecked type alias.

This is a precursor to https://crrev.com/c/1053338 which introduces
base::CheckedObserver. Existing observers will be unchecked (as they
are already). There is no behavior change with this CL.

The CL is mechanical. The bulk was done with variations on a sed script:

git grep -l ' base::ObserverList<.*> .*;' -- '*.cc' '*.h' '*.mm' | \
  xargs -IX sed -i -r 's/(^[ ]*)base::ObserverList<([^>]*)> (.*);/'\
    '\1base::ObserverList<\2>::Unchecked \3;/' X

With some manual follow-ups to get special cases.

[email protected]

Bug: 842987
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:ios-simulator-full-configs;luci.chromium.try:linux_layout_tests_slimming_paint_v2;luci.chromium.try:linux_mojo;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Idffe88e2b52f67f9226eb7b6d922070349dacc22
Reviewed-on: https://chromium-review.googlesource.com/1175511
Commit-Queue: Trent Apted <[email protected]>
Reviewed-by: Gabriel Charette <[email protected]>
Cr-Commit-Position: refs/heads/master@{#584330}
diff --git a/chrome/browser/extensions/install_tracker.h b/chrome/browser/extensions/install_tracker.h
index 60a98b03..91cdd92 100644
--- a/chrome/browser/extensions/install_tracker.h
+++ b/chrome/browser/extensions/install_tracker.h
@@ -88,7 +88,7 @@
   typedef std::map<std::string, ActiveInstallData> ActiveInstallsMap;
   ActiveInstallsMap active_installs_;
 
-  base::ObserverList<InstallObserver> observers_;
+  base::ObserverList<InstallObserver>::Unchecked observers_;
   content::NotificationRegistrar registrar_;
   PrefChangeRegistrar pref_change_registrar_;
   ScopedObserver<ExtensionRegistry, ExtensionRegistryObserver>