mojo: Move meta-API InterfaceProxy methods to a member of InterfaceProxy

Add a "$" member to InterfaceProxies and move all meta-API methods like
flushForTesting(), close(), etc. to it. This avoids collisions with
interface method names.

Bug: 849993
Change-Id: Ic311422419ba51b859fc0bbd1debcfbd5389762e
Reviewed-on: https://chromium-review.googlesource.com/c/1468851
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633394}
11 files changed