Deflake TabModelMergingTest

Deflake TabModelMergingTest by saving tab state after loading URLs and
creating tabs. Some tests started flaking after changing CTA2's activity
tab to a test URL rather than chrome://newtab. Forcing a tab save
ensures the new tab state is written to disk in #setup.

BUG=806214

Change-Id: I97a2324c5f8bd8e387d7dd73ee61cdc1633111e6
Reviewed-on: https://chromium-review.googlesource.com/902193
Commit-Queue: Theresa <twellington@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534457}

chrome/android/javatests/src/org/chromium/chrome/browser/tabmodel/TabModelMergingTest.java

@@ -140,6 +140,9 @@ public class TabModelMergingTest {
                         TabLaunchType.FROM_CHROME_UI, null);
                 mActivity2.getTabCreator(false).createNewTab(new LoadUrlParams(TEST_URL_4),
                         TabLaunchType.FROM_CHROME_UI, null);
+
+                mActivity1.saveState();
+                mActivity2.saveState();
             }
         });