CSS Font Loading: drop readonly to use Setlike functionalities

FontFaceSet.idl used readonly setlike<FontFace> to use
existing add(), delete(), and clear() implementations.

This patch drops readonly to use binding scheme to implement
writable Setlike methods.

Note that add() method returns FontFaceSet after this change.

BUG=392075

Review URL: https://codereview.chromium.org/1406153003

Cr-Commit-Position: refs/heads/master@{#354947}
7 files changed