Adding WebFallbackFont and renaming SimpleFontFamily

Step 1 of renames preceding the fix for family based fallback font selection.

BUG=382411
R=eae,behdad

Review URL: https://codereview.chromium.org/326763002

git-svn-id: svn://svn.chromium.org/blink/trunk@175858 bbb929c8-8fbe-4397-9dbb-9b2b20218538