Improve reliability of PrefetchFlowTest

There are two problems addressed here:
1. Synchronization was missing in TestOfflinePageService.sendPushMessage
2. Recent changes relating to prefetch GCM defer the call to GetGCMToken,
   which in turn results in an exception when calling
   FakeInstanceIDWithSubtype.getSubtypeAndAuthorizedEntityOfOnlyToken.

With these fixed, we (crosses fingers) can un-disable the test.

Bug: 959733
Change-Id: Ibfd29de884813606ab73a6520c627a4214d52432
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600353Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Commit-Queue: Dan H <harringtond@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657722}

chrome/android/javatests/src/org/chromium/chrome/browser/offlinepages/prefetch/PrefetchFlowTest.java

@@ -19,7 +19,6 @@ import org.chromium.base.CommandLine;
 import org.chromium.base.ContextUtils;
 import org.chromium.base.test.util.CallbackHelper;
 import org.chromium.base.test.util.CommandLineFlags;
-import org.chromium.base.test.util.DisabledTest;
 import org.chromium.base.test.util.Feature;
 import org.chromium.base.test.util.RetryOnFailure;
 import org.chromium.chrome.browser.ChromeActivity;
@@ -253,7 +252,6 @@ public class PrefetchFlowTest implements WebServer.RequestHandler {
     @Test
     @MediumTest
     @Feature({"OfflinePrefetch"})
-    @DisabledTest
     public void testPrefetchPageReadyLater() throws Throwable {
         final String url1 = "http://suggestion1.com/main";
         mSuggestionsService.addSuggestion("Suggestion 1", url1, imageUrlBase() + "suggestion1");

chrome/android/javatests/src/org/chromium/chrome/browser/offlinepages/prefetch/TestOfflinePageService.java

@@ -28,6 +28,7 @@ import org.chromium.components.offline_pages.core.prefetch.proto.OfflinePages.Pa
 import org.chromium.components.offline_pages.core.prefetch.proto.OfflinePages.PageParameters;
 import org.chromium.components.offline_pages.core.prefetch.proto.OperationOuterClass.Operation;
 import org.chromium.components.offline_pages.core.prefetch.proto.StatusOuterClass;
+import org.chromium.content_public.browser.test.util.CriteriaHelper;
 import org.chromium.content_public.browser.test.util.TestThreadUtils;
 import org.chromium.net.test.util.WebServer;
 import org.chromium.net.test.util.WebServer.HTTPHeader;
@@ -230,9 +231,11 @@ public class TestOfflinePageService {
         }
         mOperations.put(operationName, request);
         if (!writeOperationResponse(operationName, request, true, output)) {
+            synchronized (mIncompleteOperations) {
                 mIncompleteOperations.add(operationName);
             }
         }
+    }
 
     /** Send a "Forbidden by OPS" response. */
     private void respondForbidden(OutputStream output) throws IOException {
@@ -245,12 +248,32 @@ public class TestOfflinePageService {
      * name that was completed. Returns null if no bundle needs to be sent. If more than one bundle
      * needs to be sent, the first completed bundle is sent. This can be called repeatedly until no
      * more bundles are ready.
+     *
+     * This method is typically not called on the server thread, so access to members should be
+     * synchronized.
      */
     public String sendPushMessage() throws InterruptedException, TimeoutException {
-        if (mIncompleteOperations.isEmpty()) {
-            return null;
+        CriteriaHelper.pollInstrumentationThread(() -> {
+            Boolean result;
+            synchronized (mIncompleteOperations) {
+                result = !mIncompleteOperations.isEmpty();
+            }
+            return result;
+        });
+
+        String operationName;
+        synchronized (mIncompleteOperations) {
+            operationName = mIncompleteOperations.remove(0);
+        }
+        // We have to wait until Chrome gets the GCM token.
+        CriteriaHelper.pollInstrumentationThread(() -> {
+            try {
+                FakeInstanceIDWithSubtype.getSubtypeAndAuthorizedEntityOfOnlyToken();
+                return true;
+            } catch (IllegalStateException e) {
+                return false;
             }
-        String operationName = mIncompleteOperations.remove(0);
+        }, "GetGCMToken not complete", 15000, 500);
         final Pair<String, String> appIdAndSenderId =
                 FakeInstanceIDWithSubtype.getSubtypeAndAuthorizedEntityOfOnlyToken();
         final String appId = appIdAndSenderId.first;