Remove FullscreenManager accesses after it is nulled out in ChromeActivity

Crash 1:
https://crash.corp.google.com/browse?q=expanded_custom_data.ChromeCrashProto.magic_signature_1.name%3D%27%5BAndroid+Java+Exception%5D+java.lang.RuntimeException+at+org.chromium.chrome.browser.ChromeActivity.getFullscreenManager%28ChromeActivity.java%29%27+AND+product_name%3D%27Chrome_Android%27#-propertyselector,productname:1000,productversion:100,magicsignature:50,magicsignature2:50,stablesignature:50,magicsignaturesorted:50

Crash 2:
https://crash.corp.google.com/browse?q=product_name%3D%27Chrome_Android%27+AND+product.version%3D%2773.0.3683.37%27+AND+expanded_custom_data.ChromeCrashProto.channel%3D%27beta%27+AND+expanded_custom_data.ChromeCrashProto.ptype%3D%27browser%27+AND+expanded_custom_data.ChromeCrashProto.magic_signature_1.name%3D%27%5BAndroid+Java+Exception%5D+java.lang.IllegalStateException+at+org.chromium.chrome.browser.ChromeActivity.getFullscreenManager%28ChromeActivity.java%29%27

Plus potential similar crash in TWAs

Bug: 938212
Change-Id: Icd183741ea042f7d4c491a4755acc7fd4aff5050
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1503256
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Peter Conn <peconn@chromium.org>
Commit-Queue: Pavel Shmakov <pshmakov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638069}
5 files changed