Remove WebFontFamily.h

Last part of the rename from WebFontFamily to WebFallbackFont.

BUG=382411
R=jamesr,abarth

Review URL: https://codereview.chromium.org/330573007

git-svn-id: svn://svn.chromium.org/blink/trunk@176771 bbb929c8-8fbe-4397-9dbb-9b2b20218538