Make org.chromium.chromecast.base.Unit thread-safe.

It had race condition on unsynchronized access to static field. Fixed it
by using clinit (static initializer). Other typical pattern for
singletons would be "enum MyType {INSTANCE;}".

Bug: None.
Change-Id: I421e0b9f56ef1184194be2a4e083da7e06ab114c
Reviewed-on: https://chromium-review.googlesource.com/c/1333470Reviewed-by: Simeon Anfinrud <sanfin@chromium.org>
Commit-Queue: Jan Lamecki <jachor@google.com>
Cr-Commit-Position: refs/heads/master@{#607973}

chromecast/base/java/src/org/chromium/chromecast/base/Unit.java

@@ -14,10 +14,9 @@ package org.chromium.chromecast.base;
  * instantiable).
  */
 public final class Unit {
-    private static Unit sInstance;
+    private static final Unit sInstance = new Unit();
     private Unit() {}
     public static Unit unit() {
-        if (sInstance == null) sInstance = new Unit();
         return sInstance;
     }
 }