Default caret size of the empty element should match the font size

Caret size should match with the size of the font. It changes the
behavior where default caret size matches size of the box.

R=leviw
BUG=391215
TEST=Check the size of caret bound for different line height.

Review URL: https://codereview.chromium.org/376563003

git-svn-id: svn://svn.chromium.org/blink/trunk@178276 bbb929c8-8fbe-4397-9dbb-9b2b20218538