Layout Test editing/spelling/spelling-huge-text.html is flaky

Such huge text needs more time to be spell checked.
shouldBecomeEqual that was used to check spelling asynchronously allows
to pass timeout value. Change its the default value (500 ms) in this test
to 1000 ms.

BUG=298047

Review URL: https://chromiumcodereview.appspot.com/24573002

git-svn-id: svn://svn.chromium.org/blink/trunk@158339 bbb929c8-8fbe-4397-9dbb-9b2b20218538