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Before this CL, we took "partial interface mixin" a syntax error,
but Web IDL spec now accepts it to extend interface mixins.
This CL updates parser to accept it, and merges InterfaceMixin
class into Interface class.

Bug: 781257
Change-Id: I2b8584d5a1d4b8d2ffd53ba6a80c39e19feae11e
Reviewed-on: https://chromium-review.googlesource.com/1119744Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571808}