Should remove struct HitRegionOptionsInternal.

We don't need the struct because we already have IDL dictionary support.

BUG=none
TEST=existing tests

Review URL: https://codereview.chromium.org/616703003

git-svn-id: svn://svn.chromium.org/blink/trunk@183005 bbb929c8-8fbe-4397-9dbb-9b2b20218538