Add FontFaceDescriptors IDL dictionary

To follow the spec[1]. This is the first place to use IDL dictionary
execept for testing, so this CL also includes build related changes.
Existing tests(e.g. fast/css/fontface-constructor-error.html) should
cover this change.

[1] http://dev.w3.org/csswg/css-font-loading/#fontface-interface

BUG=403150

Review URL: https://codereview.chromium.org/481453003

git-svn-id: svn://svn.chromium.org/blink/trunk@181610 bbb929c8-8fbe-4397-9dbb-9b2b20218538