Mention LifetimeAssert in finalizer styleguide

Change-Id: Ib1c8c8f6cb1f74c32c543ae84844e634b26fcb79
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422428Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809144}

styleguide/java/java.md

@@ -121,7 +121,8 @@ Custom finalizers:
 * causes additional garbage collector jank.
 
 Classes that need destructor logic should provide an explicit `destroy()`
-method.
+method. Use [LifetimeAssert](https://chromium.googlesource.com/chromium/src/+/master/base/android/java/src/org/chromium/base/LifetimeAssert.java)
+to ensure in debug builds and tests that `destroy()` is called.
 
 ### Other Android Support Library Annotations
 * Use them! They are [documented here](https://developer.android.com/studio/write/annotations).