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This is a stripped down incarnation of Service Manager's BinderMap type.
Unlike that type, there is no intent here to support the complexity of
variadic binder arguments since such use cases are generally now limited
to the Content layer or above.

This is a straightforward mapping of interface name to callback + task
runner.

At first this will at least be used by Blink and various Content child
process types to expose interfaces to the browser through a common path
in ChildThreadImpl.

Bug: 977637
Change-Id: I7aa7cf40995b1898090202bd4e4377285c776421
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1905047
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714815}