[Chromoting] Use MockConnection for bump scrolling browser test.

[Chromoting] Use MockConnection for bump scrolling browser test.

This CL uses the MockConnection for bump scrolling browser test.
This will make the test more resilient to network failures.

BUG=478331
Committed: https://crrev.com/57cdc723f71b691b10ddc8f10f9a56897acb6886
Cr-Commit-Position: refs/heads/master@{#326980}

[kelvinp, 2015-04-25 00:24:53]

kelvinp@chromium.org changed reviewers:
+ jamiewalch@chromium.org

[kelvinp, 2015-04-25 00:25:55]

PTAL

[Jamie, 2015-04-25 00:31:42]

lgtm

[kelvinp, 2015-04-25 22:15:45]

The CQ bit was checked by kelvinp@chromium.org

[kelvinp, 2015-04-25 22:15:45]

The patchset sent to the CQ was uploaded after l-g-t-m from
jamiewalch@chromium.org
Link to the patchset: https://codereview.chromium.org/1086563005/#ps20001
(title: "Rebase")

[commit-bot: I haz the power, 2015-04-25 22:16:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1086563005/20001

[commit-bot: I haz the power, 2015-04-25 23:08:54]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-25 23:09:42]

Patchset 2 (id:??) landed as
https://crrev.com/57cdc723f71b691b10ddc8f10f9a56897acb6886
Cr-Commit-Position: refs/heads/master@{#326980}