Cleanup <static_cast>: Switch to toFoo for Accessibility Objects.

To enforce more strictly the avoidance of static_cast on render objects, existing usage of static_cast needs to be corrected first. 

R=tkent

BUG=None

TEST=None; no behavior changes;

Review URL: https://chromiumcodereview.appspot.com/24752002

git-svn-id: svn://svn.chromium.org/blink/trunk@158399 bbb929c8-8fbe-4397-9dbb-9b2b20218538