Disable flaky showcase tests

ios/showcase/tab/sc_tab_egtest.mm

Index: ios/showcase/tab/sc_tab_egtest.mm
diff --git a/ios/showcase/tab/sc_tab_egtest.mm b/ios/showcase/tab/sc_tab_egtest.mm
index e4190610ef5f66998cdc02bce74ef38117c7864a..5da16e7658991a777444685fa27925e470cd4cdb 100644
--- a/ios/showcase/tab/sc_tab_egtest.mm
+++ b/ios/showcase/tab/sc_tab_egtest.mm
@@ -18,13 +18,15 @@ @interface SCTabTestCase : ShowcaseTestCase
 @implementation SCTabTestCase
 
 // Tests launching TopToolbarTabViewController.
-- (void)testLaunchWithTopToolbar {
+// TODO(crbug.com/710662): re-enable this test.
+- (void)FLAKY_testLaunchWithTopToolbar {
   showcase_utils::Open(@"TopToolbarTabViewController");
   showcase_utils::Close();
 }
 
 // Tests launching BottomToolbarTabViewController.
-- (void)testLaunchWithBottomToolbar {
+// TODO(crbug.com/710662): re-enable this test.
+- (void)FLAKY_testLaunchWithBottomToolbar {
   showcase_utils::Open(@"BottomToolbarTabViewController");
   showcase_utils::Close();
 }