Android: Fix NPE in gesture navigation.

We evaluate gesture navigation enabled status every time Chrome app is
resumed in order to deal with the system gesture navigation mode
change while the app was in background. When testing, the parent view
(CompositorViewHolder) is always in attached state when the activity
is resumed, but it can happen that it is in detached state as reported
in the bug. This caused an exception, since WindowInsets will be null
in that state.

This CL skips the check if the state is queried while the view is
detached from its window. The only scenario that could potentially
be affected is when user switched the system gesture navigation on
the settings and resume the app later. But experiment shows that
this re-creates ChromeActivity and attaches the views before
resuming, and the mode change rarely happens in the field. So the
change is not likely to be risky.

Bug: 1045942
Change-Id: I7ac11796737b1f355786c65f309182b107c39f20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2029547Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737552}

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

@@ -148,7 +148,10 @@ public class HistoryNavigationCoordinator implements InsetObserverView.WindowIns
         if (Build.VERSION.SDK_INT < Build.VERSION_CODES.Q) {
             return true;
         } else {
-            assert mCompositorViewHolder.isAttachedToWindow();
+            // Preserve the previous enabled status if queried when the view is in detached state.
+            if (mCompositorViewHolder == null || !mCompositorViewHolder.isAttachedToWindow()) {
+                return mEnabled;
+            }
             Insets insets = mCompositorViewHolder.getRootWindowInsets().getSystemGestureInsets();
             return insets.left == 0 && insets.right == 0;
         }