Adding MeterRole in blink to support MSAA+IA2 and AX roles for meter tag in Chromium.

According to latest editor's draft http://rawgit.com/w3c/html-api-map/master/index.html
meter tag should have MSAA+IA2 Role as ROLE_SYSTEM_PROGRESSBAR and
AX role as AXGroup and Role description as "group"

BUG=421376

Review URL: https://codereview.chromium.org/652663003

git-svn-id: svn://svn.chromium.org/blink/trunk@183638 bbb929c8-8fbe-4397-9dbb-9b2b20218538