Reland 149510 - Allow filters in accessibility tests to specify which attributes to check.

Reland 149510 - Allow filters in accessibility tests to specify which attributes to check.

Rolled this out in hopes of stopping some crashes, but it didn't help, so relanding.

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

TBR=pkasting@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=149573