Use shouldBecomeEqual in asynchronous spellchecking tests.

Every asynchronous spellchecking layout test waits for spelling or grammar
markers using setTimeout. It duplicates the code, moreover the implementation
differs in every test.

Use shouldBecomeEqual helper (introduced in WebKit's r143860) to test
spellchecking asynchronously.

In grammar-paste.html, dump text of destination element instead of source one
to show what is really tested. It stops outputting that the input and textarea
elements can store html tags, for example,
PASS INPUT has a marker on 'I have a<b>n ki</b>wi'.

This is mostly merge of WebKit changeset: http://trac.webkit.org/changeset/156137

In addition, adjust the latest Blink specific spellchecking tests to start using
shouldBecomeEqual apart from spellcheck-editable-on-focus.html due to its flakiness.

Turn on asynchronous spellchecking at the beginning of spellcheck-paste-disabled.html
and spellcheck-paste-continuous-disabled.html. Previously, the feature was enabled in
the second part of the tests and first one used synchronous path of spell checking which
is going to be removed.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23534071

git-svn-id: svn://svn.chromium.org/blink/trunk@158283 bbb929c8-8fbe-4397-9dbb-9b2b20218538