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Ensure there's no layout pending before accessing WebAXObject from TestRunner.

It's not safe to call WebAXObject functions when there's a layout pending.
Chrome never does this, it only calls WebAXObject from
renderer_accessibility_complete, and it makes sure to update layout before
accessing the accessibility tree.

TestRunner didn't have the same protections, so this just explicitly updates
the layout before any accessibility call.

BUG=393829

Review URL: https://codereview.chromium.org/469803003

Cr-Commit-Position: refs/heads/master@{#289489}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289489 0039d316-1c4b-4281-b951-d872f2087c98