[Android] Fix subthread native test execution race condition.

testing/android/native_test/java/src/org/chromium/native_test/NativeTest.java

Index: testing/android/native_test/java/src/org/chromium/native_test/NativeTest.java
diff --git a/testing/android/native_test/java/src/org/chromium/native_test/NativeTest.java b/testing/android/native_test/java/src/org/chromium/native_test/NativeTest.java
index 4352205a23aa611bda3d7fc587056f336dfc1810..3e27274c40e610295fd5891df874efc58395f5c8 100644
--- a/testing/android/native_test/java/src/org/chromium/native_test/NativeTest.java
+++ b/testing/android/native_test/java/src/org/chromium/native_test/NativeTest.java
@@ -128,23 +128,40 @@ public class NativeTest {
     }
 
     public void postStart(final Activity activity, boolean forceRunInSubThread) {
+        final Runnable runTestsTask = new Runnable() {
+            @Override
+            public void run() {
+                runTests(activity);
+            }
+        };
+
         if (mRunInSubThread || forceRunInSubThread) {
-            // Create a new thread and run tests on it.
-            new Thread() {
+            // Post a task that posts a task that creates a new thread and runs tests on it.
+
+            // On L and M, the system posts a task to the main thread that prints to stdout
+            // from android::Layout (https://goo.gl/vZA38p). Chaining the subthread creation
+            // through multiple tasks executed on the main thread ensures that this task
+            // runs before we start running tests s.t. its output doesn't interfere with
+            // the test output. See crbug.com/678146 for additional context.
+
+            final Handler handler = new Handler();
+            final Runnable startTestThreadTask = new Runnable() {
                 @Override
                 public void run() {
-                    runTests(activity);
+                    new Thread(runTestsTask).start();
                 }
-            }.start();
-        } else {
-            // Post a task to run the tests. This allows us to not block
-            // onCreate and still run tests on the main thread.
-            new Handler().post(new Runnable() {
+            };
+            final Runnable postTestStarterTask = new Runnable() {
                 @Override
                 public void run() {
-                    runTests(activity);
+                    handler.post(startTestThreadTask);
                 }
-            });
+            };
+            handler.post(postTestStarterTask);
+        } else {
+            // Post a task to run the tests. This allows us to not block
+            // onCreate and still run tests on the main thread.
+            new Handler().post(runTestsTask);
         }
     }