Change synthetic bold/italic test to pixel test

We currently have two tests that assume that web fonts and system fonts
render synthetic bold/italic identically. This is not necessarily true
and also doesn't actually test whether synthetic bold/italic works.

Remove synthetic bold/italic tests from font-face-multiple-faces and
change font-face-synthetic-bold-italic to be a pixel test.

BUG=302846
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/25505009

git-svn-id: svn://svn.chromium.org/blink/trunk@158683 bbb929c8-8fbe-4397-9dbb-9b2b20218538