Revert 163929 "Generate an accessibility notification on modal d..."

This change is making several content_browsertests accessibility time
out on Win7 Dbg. On my Mac book, I saw some similar failures for:
DumpAccessibilityTreeTest.AccessibilityModalDialogClosed
DumpAccessibilityTreeTest.AccessibilityModalDialogClosed

> Generate an accessibility notification on modal dialog open/close.
> 
> BUG=324392
> 
> Review URL: https://codereview.chromium.org/112303004

TBR=dominicc@chromium.org

Review URL: https://codereview.chromium.org/107653004

git-svn-id: svn://svn.chromium.org/blink/trunk@163959 bbb929c8-8fbe-4397-9dbb-9b2b20218538