Adds accessibility attributes for "embed", "object" and "applet" tags.

R=dmazzoni,dtseng
BUG=348259

Review URL: https://codereview.chromium.org/231883009

git-svn-id: svn://svn.chromium.org/blink/trunk@171680 bbb929c8-8fbe-4397-9dbb-9b2b20218538