java.md: Add text about finalizers to style guide

Based on java@ discussion:
https://groups.google.com/a/chromium.org/d/topic/java/9WZLCc14oss/discussion

Bug: 149440
Change-Id: Ie36166fe2282af564daafb13fbcb91af1a4ab6c1
Reviewed-on: https://chromium-review.googlesource.com/c/1344889
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611176}

styleguide/java/java.md

@@ -114,6 +114,18 @@ if (org.chromium.base.BuildConfig.DCHECK_IS_ON) {
 }
 ```
 
+### Finalizers
+In line with [Google's Java style guide](https://google.github.io/styleguide/javaguide.html#s6.4-finalizers),
+never override `Object.finalize()`.
+
+Custom finalizers:
+* are called on a background thread, and at an unpredicatble point in time,
+* swallow all exceptions (asserts won't work),
+* causes additional garbage collector jank.
+
+Classes that need destructor logic should provide an explicit `destroy()`
+method.
+
 ### Other Android Support Library Annotations
 * Use them! They are [documented here](https://developer.android.com/studio/write/annotations).
   * They generally improve readability.