Protect against double destruction

No definitive repro, but found in several large
automated test runs with rapid activity on device.
All protect this remaining member against access
after destruction.

Bug: b/163868029
Test: Local execution, no regression
Change-Id: Icaf5e73f10f75e6edcf414c830b8eb99611b4b27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2503637Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Commit-Queue: Brett Kilty <brettk@google.com>
Cr-Commit-Position: refs/heads/master@{#822051}

base/android/java/src/org/chromium/base/UnownedUserDataHost.java

@@ -248,6 +248,11 @@ public final class UnownedUserDataHost {
     public void destroy() {
         mThreadChecker.assertOnValidThread();
 
+        if (isDestroyed()) {
+            // Protect against potential races.
+            return;
+        }
+
         // Create a shallow copy of all keys to ensure each held object can safely remove itself
         // from the map while iterating over their keys.
         Set<UnownedUserDataKey<?>> keys = new HashSet<>(mUnownedUserDataMap.keySet());