Mark testAppInstalledModalNativeAppBannerBrowserTab flaky.

TBR: dominickn@chromium.org
Bug: 1139496
Change-Id: I65f4c3e974ec832e102803213fcdb4c1c7ff6079
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2481744Reviewed-by: Sky Malice <skym@chromium.org>
Commit-Queue: Sky Malice <skym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818150}

chrome/android/javatests/src/org/chromium/chrome/browser/banners/AppBannerManagerTest.java

@@ -439,6 +439,7 @@ public class AppBannerManagerTest {
     @Test
     @SmallTest
     @Feature({"AppBanners"})
+    @FlakyTest(message = "https://crbug.com/1139496")
     public void testAppInstalledModalNativeAppBannerBrowserTab() throws Exception {
         triggerModalNativeAppBanner(mTabbedActivityTestRule,
                 WebappTestPage.getNonServiceWorkerUrlWithManifestAndAction(mTestServer,