CSS Font Loading: drop readonly to use Setlike functionalities

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
Committed: https://crrev.com/570fb77ac846fb6f52c67e14245e254d1ae9db43
Cr-Commit-Position: refs/heads/master@{#354947}

[Takashi Toyoshima, 2015-10-19 09:00:40]

toyoshim@chromium.org changed reviewers:
+ ksakamoto@chromium.org

[Takashi Toyoshima, 2015-10-19 09:30:39]

Sakamoto-san, can you take a look?
This is the second and the last patch to make FontFaceSet setlike<FontFaceSet>.

[Kunihiko Sakamoto, 2015-10-19 09:48:20]

lgtm

[Takashi Toyoshima, 2015-10-19 13:29:07]

toyoshim@chromium.org changed reviewers:
+ haraken@chromium.org, tkent@chromium.org

[Takashi Toyoshima, 2015-10-19 13:29:08]

haraken@chromium.org: Please review changes in core/.
tkent@chromium.org: Please review changes in FontFaceSet.idl

[Takashi Toyoshima, 2015-10-19 13:30:36]

cc: philipj since this change fixes a TODO(philipj)

[haraken, 2015-10-19 14:16:18]

core/ LGTM

[tkent, 2015-10-19 23:21:57]

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

The change is trivial.  LGTM

[Takashi Toyoshima, 2015-10-20 01:06:37]

The CQ bit was checked by toyoshim@chromium.org

[commit-bot: I haz the power, 2015-10-20 01:07:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1406153003/80001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1406153003/80001

[commit-bot: I haz the power, 2015-10-20 01:14:04]

Committed patchset #5 (id:80001)

[commit-bot: I haz the power, 2015-10-20 01:15:25]

Patchset 5 (id:??) landed as
https://crrev.com/570fb77ac846fb6f52c67e14245e254d1ae9db43
Cr-Commit-Position: refs/heads/master@{#354947}