Fix flaky autofill tests in //ios/web_view.

These tests were flaky because submitting a form causes a navigation
on top of loading the test page. These two navigations happening at the
same time can cause some race conditions ending in a DCHECK.

By directing the form to 'about:blank', it avoids the extra navigation.

Bug: 834660,842122
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: If535a18fdc277d0c6766967cdbeea2f85fa415dc
Reviewed-on: https://chromium-review.googlesource.com/1056411
Commit-Queue: John Wu <jzw@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558335}

ios/web_view/test/web_view_autofill_inttest.mm

@@ -33,7 +33,8 @@ NSString* const kTestFieldValue = @"FieldValue";
 NSString* const kTestSubmitID = @"SubmitID";
 NSString* const kTestFormHtml =
     [NSString stringWithFormat:
-                  @"<form name='%@' id='%@'>"
+                  // Direct form to about:blank to avoid unnecessary navigation.
+                  @"<form action='about:blank' name='%@' id='%@'>"
                    "<input type='text' name='%@' id='%@'/>"
                    "<input type='submit' id='%@'/>"
                    "</form>",