Layout Test editing/spelling/spellcheck-editable-on-focus.html is flaky

Do not use 'shouldBe' helper to check markers when asynchronous spell checking
is being used. Even if they should not have markers, we need to wait for them.

According to r158283 use shouldBecomeEqual. However, the spelling markers are
verified synchronously. As a result, we will check next marker only when the
current one appears. It allows us to set focus on elements and make sure
that it does not influence the previous expectations (where markers should not be
present).

BUG=278318

Review URL: https://codereview.chromium.org/25507002

git-svn-id: svn://svn.chromium.org/blink/trunk@161840 bbb929c8-8fbe-4397-9dbb-9b2b20218538