Refactoring inline_spelling_markers.html to use async path for spellcheck

This change mostly adapts inline_spelling_markers.html to use asynchronous
path for spellchecking which will be enabled by default.
To achieve this, shouldBecomeEqual helper has been used.

Additionally, make cross platform text expectation by dumping
spelling/grammar markers instead of whole tree which in terms
of spellchecking says nothing.

Enabling unified text checker and adjusting MockGrammarCheck.cpp
allow us to restore grammar checking in inline_spelling_markers.html.

Remove unnecessary new lines in div elements so position of markers
can be given from 0.

BUG=295722

Review URL: https://codereview.chromium.org/49613006

git-svn-id: svn://svn.chromium.org/blink/trunk@161436 bbb929c8-8fbe-4397-9dbb-9b2b20218538