Android: Fix NPE in gesture navigation

Added a null check in the public API of BottomSheetContent impl in
gesture navigation. This can be invoked before BottomSheetController
is available.

Bug: 1002385
Change-Id: I991c3caed0a118b89178321bd86f5c3e82e406ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1795523Reviewed-by: Donn Denman <donnd@chromium.org>
Commit-Queue: Donn Denman <donnd@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695389}

chrome/android/java/src/org/chromium/chrome/browser/gesturenav/NavigationSheetCoordinator.java

@@ -205,6 +205,7 @@ class NavigationSheetCoordinator implements BottomSheetContent, NavigationSheet
 
     @Override
     public boolean isHidden() {
+        if (mBottomSheetController.get() == null) return true;
         return getTargetOrCurrentState() == SheetState.HIDDEN;
     }