Add an initialize function to match other impl objects and simplify the
creation of DeviceImpls.

BUG=742034

Change-Id: I569c8252d5d23c8af98835f6c08e7a3b640fc3f3
Reviewed-on: https://chromium-review.googlesource.com/854626Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>