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This patch adds a new method setSpellCheckResolvedCallback to test
runner, so that layout tests can run script when a spell check request
is resolved, which can be used to inspect spelling markers in a
better manner. It also adds removeSpellCheckResolvedCallback for clearing
a previously set callback.

Currently layout tests can only set a timeout to wait for markers to appear.
Follow-up patches will utilize this new method in layout tests.

BUG=674819

Review-Url: https://codereview.chromium.org/2587823003
Cr-Commit-Position: refs/heads/master@{#439725}