While we do support variations, our format specifier support was not
updated yet. Move all of that to FontCustomPlatformData and add a WPT
test to cover common supported format specifiers.

Test: external/wpt/css/css-fonts/format-specifiers-variations.html
Bug: 804466
Change-Id: I06d7af65cf74e13cec49dc279efb8ffc2b56dda9
Reviewed-on: https://chromium-review.googlesource.com/926007
Reviewed-by: Koji Ishii <[email protected]>
Commit-Queue: Dominik Röttsches <[email protected]>
Cr-Commit-Position: refs/heads/master@{#537799}