Adding PreRole in blink side to expose HTML pre tag with correct MSAA+IA2.

This is blink side CL to expose HTML pre tag in chromium side with proper MSAA+IA2 role.

BUG=426053

Review URL: https://codereview.chromium.org/671003002

git-svn-id: svn://svn.chromium.org/blink/trunk@184300 bbb929c8-8fbe-4397-9dbb-9b2b20218538