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This depends on: http://codereview.chromium.org/10662003/

Based on the HTML to Platform Accessibility APIs Implementation Guide
(http://www.w3.org/TR/html-aapi/), add some new tests to assert the
correct behavior for computing the text alternative for a
checkbox, button, and text field, then fix the implementation to
match the test.

BUG=124314
TBR=dtseng

Review URL: https://chromiumcodereview.appspot.com/10823073

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@150070 0039d316-1c4b-4281-b951-d872f2087c98