Connect to DeviceManager lazily in WebUSBClientImpl.

Connect to DeviceManager lazily in WebUSBClientImpl.

Every WebLocalFrameImpl results in the creation of a WebUSBClientImpl so
this patch delays the connection to the embedder's DeviceManager
implementation until it is actually needed. This means it will end up
being connected when the navigator.usb object is first accessed by
Javascript.

This avoids exposing everyone who has enabled the experimental web
platform features flag to instabilities in this new feature and will
generally reduce resource utilization for unused features when this is
shipped.

BUG=538339
Committed: https://crrev.com/fd6c723ff1b86bdc3c8036017eae3bbd80127840
Cr-Commit-Position: refs/heads/master@{#353909}

[Reilly Grant (use Gerrit), 2015-10-13 22:17:51]

reillyg@chromium.org changed reviewers:
+ rockot@chromium.org

[Reilly Grant (use Gerrit), 2015-10-13 22:17:52]

Please take a look.

[Ken Rockot(use gerrit already), 2015-10-13 22:20:22]

lgtm

[Reilly Grant (use Gerrit), 2015-10-13 22:56:27]

The CQ bit was checked by reillyg@chromium.org

[commit-bot: I haz the power, 2015-10-13 22:57:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1404043002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1404043002/1

[commit-bot: I haz the power, 2015-10-13 23:43:51]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-13 23:45:08]

Patchset 1 (id:??) landed as
https://crrev.com/fd6c723ff1b86bdc3c8036017eae3bbd80127840
Cr-Commit-Position: refs/heads/master@{#353909}