Address a few more flakinesses related to Ahem font.

overflow-scroll-rtl.html was an interesting case, because a wrong baseline
image (i.e. Ahem font not loaded) was checked in for this test. The image
is rebaselined to the correct one. The test is flaky nonetheless, so the test
expectation is also added.

BUG=392046
TBR=pilgrim@chromium.org

Review URL: https://codereview.chromium.org/371293002

git-svn-id: svn://svn.chromium.org/blink/trunk@177648 bbb929c8-8fbe-4397-9dbb-9b2b20218538