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Add a OneTimeMessagingHandler to handle sending one-time messages (such
as those from chrome.runtime.sendMessage) with native bindings. These
messages open a message channel, similar to the long-lived connections
of chrome.runtime.connect(), but the channel is only open for a single
message-and-response cycle.

The OneTimeMessagingHandler is responsible for creating and maintaining
these message channels, and cleaning them up when complete.

Add tests for the same.

Note: currently, these constructs are still only used in tests. A later
CL will hook them up to the bindings system.

Bug: 653596
Change-Id: Ibeae3bda73b66306a85b13749f1cab1a1edfb33b
Reviewed-on: https://chromium-review.googlesource.com/675305
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506932}