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Add a StartPromiseBasedRequest() method to APIRequestHandler to begin an
API request that returns a Promise.  The promise will be fulfilled or
rejected once the API returns. An API error (i.e., one that would
normally be presented in chrome.runtime.lastError) rejects the promise,
and otherwise it is resolved with the result of the API call.  Add
unittests for the same.

Since promises can only be resolved with a single value, only APIs that
return a single result are supported.

Note this only adds support in the APIRequestHandler class; this is not
currently hooked up to any APIs, and cannot be used directly from
extensions yet.

Bug: 978538

Change-Id: I2bcd7c48b82656187d8303f088211b0557fc0ac4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626654Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676985}