[iOS][Flaky] Mark KeyboardObserverTestCase.testKeyboardDidHide flaky

This test was failing frequently, see linked bug for details.

TBR=javierrobles@chromium.org

Bug: 914374
Change-Id: I6cf9df559992c594258314fbb7e736a6da9c023d
Reviewed-on: https://chromium-review.googlesource.com/c/1373766Reviewed-by: Friedrich Horschig [CET] <fhorschig@chromium.org>
Commit-Queue: Friedrich Horschig [CET] <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615918}

ios/chrome/browser/ui/autofill/manual_fill/keyboard_observer_egtest.mm

@@ -149,7 +149,8 @@ void TapOnWebElementWithID(const std::string& elementID) {
 }
 
 // Tests that when the keyboard actually dismiss the right callback is done.
-- (void)testKeyboardDidHide {
+// TODO(crbug.com/914374): Address flakiness and reenable.
+- (void)DISABLED_testKeyboardDidHide {
   // Brings up the keyboard by tapping on one of the form's field.
   TapOnWebElementWithID(kFormElementID1);