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This may have caused the Win 7 WebKit canary bot to begin crashing; see
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=accessibility%2Floading-iframe-updates-axtree.html
If rolling this out does not fix the bot, I will put it back in later tonight.

With this change, a DumpAccessibilityTree test can specify filters to
control which attributes are printed. This keeps each test expectation file
small and readable, while making it easy to test for all sorts of attributes,
including obscure ones.

Each platform will have a few attributes that print by default, like the
role and name. HTML files add rules of this form to override the defaults:

  @MAC-DENY:subrole*
  @MAC-DENY:value*
  @MAC-ALLOW:description*

You can also specify @MAC-ALLOW:* or @WIN-ALLOW:* if you want to dump everything - helpful
during development sometimes.

BUG=124314


Review URL: https://chromiumcodereview.appspot.com/10662003

TBR=dmazzoni@chromium.org
Review URL: https://chromiumcodereview.appspot.com/10837065

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@149550 0039d316-1c4b-4281-b951-d872f2087c98