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This CL implements mojom client on blink side.
 - FaceDetector::detect() sends an image to the browser through shared_buffer;
 - FaceDetector::onDetectFace() receives a list of boundingBoxes from Chrome.
A mock mojom server is added to test the connection.

This CL also reduces the API surface by removing Detector and DetectedObject
interface, so that the hierarchy is simpler and cleaner.

BUG=646083
TEST=Pass LayoutTest detectface.html

Review-Url: https://codereview.chromium.org/2369693002
Cr-Commit-Position: refs/heads/master@{#422729}