Automated g4 rollback of changelist 191128111.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=191834511

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/SimpleCache.java

@@ -93,6 +93,10 @@ public final class SimpleCache implements Cache {
    * @param index The CachedContentIndex to be used.
    */
   /*package*/ SimpleCache(File cacheDir, CacheEvictor evictor, CachedContentIndex index) {
+    if (!lockFolder(cacheDir)) {
+      throw new IllegalStateException("Another SimpleCache instance uses the folder: " + cacheDir);
+    }
+
     this.cacheDir = cacheDir;
     this.evictor = evictor;
     this.index = index;

library/core/src/test/java/com/google/android/exoplayer2/upstream/cache/SimpleCacheTest.java

@@ -296,17 +296,17 @@ public class SimpleCacheTest {
     }
   }
 
-  //   @Test
-  //   public void testMultipleSimpleCacheWithSameCacheDirThrowsException() throws Exception {
-  //     new SimpleCache(cacheDir, new NoOpCacheEvictor());
-  //
-  //     try {
-  //       new SimpleCache(cacheDir, new NoOpCacheEvictor());
-  //       assertWithMessage("Exception was expected").fail();
-  //     } catch (IllegalStateException e) {
-  //       // Expected. Do nothing.
-  //     }
-  //   }
+  @Test
+  public void testMultipleSimpleCacheWithSameCacheDirThrowsException() throws Exception {
+    new SimpleCache(cacheDir, new NoOpCacheEvictor());
+
+    try {
+      new SimpleCache(cacheDir, new NoOpCacheEvictor());
+      assertWithMessage("Exception was expected").fail();
+    } catch (IllegalStateException e) {
+      // Expected. Do nothing.
+    }
+  }
 
   @Test
   public void testMultipleSimpleCacheWithSameCacheDirDoesNotThrowsExceptionAfterRelease()