Fix crash caused by stylus battery delegate

Crash occurs if a stylus battery update
comes in before the `battery_update_callback_` is set in
the `BatteryView` constructor.
Adding protective check to make sure we don't use the callback
before it is set.

(cherry picked from commit 45f13e7)

Bug: 1184918
Change-Id: I1a9496986f2ca168648a6400734b26104c0c1377
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2738919
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#860328}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2739655
Cr-Commit-Position: refs/branch-heads/4430@{#210}
Cr-Branched-From: e5ce7dc-refs/heads/master@{#857950}

ash/system/palette/stylus_battery_delegate.cc

@@ -95,7 +95,8 @@ void StylusBatteryDelegate::OnAddingBattery(
   battery_level_ = battery.level;
   battery_charge_status_ = battery.charge_status;
   last_update_timestamp_ = battery.last_update_timestamp;
-  battery_update_callback_.Run();
+  if (battery_update_callback_)
+    battery_update_callback_.Run();
 }
 
 void StylusBatteryDelegate::OnRemovingBattery(
@@ -106,7 +107,8 @@ void StylusBatteryDelegate::OnUpdatedBatteryLevel(
   battery_level_ = battery.level;
   battery_charge_status_ = battery.charge_status;
   last_update_timestamp_ = battery.last_update_timestamp;
-  battery_update_callback_.Run();
+  if (battery_update_callback_)
+    battery_update_callback_.Run();
 }
 
 }  // namespace ash