Fix the browser test RestoreSessionForThreeUsers flaky issue.

The browser test RestoreSessionForThreeUsers might be finished very
fast, before FullRestoreService::Init is called after getting the full
restore data. app_launch_handler_ could be reset as null in
FullRestoreService::Observe, because the system shutdown process has
started. So calling app_launch_handler_->IsRestoreDataLoaded() in
FullRestoreService::Init might cause crash, because app_launch_handler_
could be reset as null.

Modify FullRestoreService::Init to check whether app_launch_handler_ is
null to prevent crash in tests.

BUG=1249033

Change-Id: Id8cad72204d3215f15b811256e20e82fb4f25011
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3216889
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#930244}