[Screenshot] set mWindowAndroid to null on share sheet destroy

mWindowAndroid==null is overloaded to indicate if ShareSheetController
is registered as an ActivityStateObserver. Setting it to null here
maintains that relationship.

Bug: 1093365
Change-Id: Ia33c96e57e0504ce50fb6cefcb34b14fdcf25136
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2298176Reviewed-by: Kyle Milka <kmilka@chromium.org>
Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Jeffrey Cohen <jeffreycohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788651}

chrome/browser/share/android/java/src/org/chromium/chrome/browser/share/share_sheet/ShareSheetCoordinator.java

@@ -86,6 +86,7 @@ public class ShareSheetCoordinator
     protected void destroy() {
         if (mWindowAndroid != null) {
             mWindowAndroid.removeActivityStateObserver(this);
+            mWindowAndroid = null;
         }
     }