[Cronet] Regression test for DCHECK when histograms read on another thread

components/cronet/android/test/javatests/src/org/chromium/net/CronetUrlRequestContextTest.java

Index: components/cronet/android/test/javatests/src/org/chromium/net/CronetUrlRequestContextTest.java
diff --git a/components/cronet/android/test/javatests/src/org/chromium/net/CronetUrlRequestContextTest.java b/components/cronet/android/test/javatests/src/org/chromium/net/CronetUrlRequestContextTest.java
index e0f7186c14df82134c55cc15ae153caaaf79eb29..7c9852c9ba409012294474ba38512b25b96252d3 100644
--- a/components/cronet/android/test/javatests/src/org/chromium/net/CronetUrlRequestContextTest.java
+++ b/components/cronet/android/test/javatests/src/org/chromium/net/CronetUrlRequestContextTest.java
@@ -1145,16 +1145,27 @@ public class CronetUrlRequestContextTest extends CronetTestBase {
     public void testGetGlobalMetricsDeltas() throws Exception {
         final CronetTestFramework testFramework = startCronetTestFramework();
 
-        byte delta1[] = testFramework.mCronetEngine.getGlobalMetricsDeltas();
+        final byte delta1[] = testFramework.mCronetEngine.getGlobalMetricsDeltas();
 
-        TestUrlRequestCallback callback = new TestUrlRequestCallback();
+        final TestUrlRequestCallback callback = new TestUrlRequestCallback();
         UrlRequest.Builder builder = testFramework.mCronetEngine.newUrlRequestBuilder(
                 mUrl, callback, callback.getExecutor());
         builder.build().start();
-        callback.blockForDone();
-        byte delta2[] = testFramework.mCronetEngine.getGlobalMetricsDeltas();
-        assertTrue(delta2.length != 0);
-        assertFalse(Arrays.equals(delta1, delta2));
+        // Fetch deltas on a different thread the second time to make sure this is permitted.
+        // See crbug.com/719448
+        final ConditionVariable cv = new ConditionVariable();
+        new Thread(new Runnable() {
+            @Override
+            public void run() {
+                callback.blockForDone();
+                byte delta2[] = testFramework.mCronetEngine.getGlobalMetricsDeltas();
+                assertTrue(delta2.length != 0);

[mef, 2017/05/09 15:13:26]

AFAIK JUnit assertions only work properly on the main test thread. 
Can we collect delta2 on worker thread, but perform asserts after cv.block()?

[pauljensen, 2017/05/09 15:55:54]

Done.

+                assertFalse(Arrays.equals(delta1, delta2));
+                cv.open();
+            }
+        })
+                .start(); // TODO(pauljensen): weird spaces crbug.com/719964
+        cv.block();
     }
 
     @SmallTest