Disable EarlGrey web shell tests that fail on device.

ios/web/shell/test/navigation_egtest.mm

Index: ios/web/shell/test/navigation_egtest.mm
diff --git a/ios/web/shell/test/navigation_egtest.mm b/ios/web/shell/test/navigation_egtest.mm
index 8f7f818d2c5291ddaf06d662978fff218ba99d7d..00839f661bdc7bda73055c23340280e9a34712a1 100644
--- a/ios/web/shell/test/navigation_egtest.mm
+++ b/ios/web/shell/test/navigation_egtest.mm
@@ -44,8 +44,14 @@
       assertWithMatcher:grey_notNil()];
 }
 
+// TODO(crbug.com/675015): Re-enable this test on device.
+#if TARGET_IPHONE_SIMULATOR
+#define MAYBE_testNavigationBackAndForward testNavigationBackAndForward
+#else
+#define MAYBE_testNavigationBackAndForward FLAKY_testNavigationBackAndForward
+#endif
 // Tests the back and forward button after entering two URLs.
-- (void)testNavigationBackAndForward {
+- (void)MAYBE_testNavigationBackAndForward {
   // Create map of canned responses and set up the test HTML server.
   std::map<GURL, std::string> responses;
   const GURL URL1 = web::test::HttpServer::MakeUrl("http://firstURL");
@@ -85,8 +91,16 @@
       assertWithMatcher:grey_notNil()];
 }
 
+// TODO(crbug.com/675015): Re-enable this test on device.
+#if TARGET_IPHONE_SIMULATOR
+#define MAYBE_testNavigationBackAndForwardAfterFragmentLink \
+  testNavigationBackAndForwardAfterFragmentLink
+#else
+#define MAYBE_testNavigationBackAndForwardAfterFragmentLink \
+  FLAKY_testNavigationBackAndForwardAfterFragmentLink
+#endif
 // Tests back and forward navigation where a fragment link is tapped.
-- (void)testNavigationBackAndForwardAfterFragmentLink {
+- (void)MAYBE_testNavigationBackAndForwardAfterFragmentLink {
   // Create map of canned responses and set up the test HTML server.
   std::map<GURL, std::string> responses;
   const GURL URL1 = web::test::HttpServer::MakeUrl("http://fragmentLink");