[ios] Fixes a crash when the app is shut down without being started.

If a user force-quits the app during launch (while the splash screen is
still visible), it's possible for the app to receive
applicationWillTerminate: without first receiving
application:didFinishLaunchingWithOptions:. In this scenario, invoking
the normal shutdown logic will lead to crashes, because the app was
never started up.

This CL adds logic to detect this case and return early from
applicationWillTerminate.

BUG=1115863,983504,1086396

Change-Id: I453f0ecae35a4431dc51490c6e38bbfd0d224f01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367986
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800986}