DeviceUtils.PushChangedFiles(): Don't run mkdir when it's not necessary

DeviceUtils.PushChangedFiles(): Don't run mkdir when it's not necessary

Shaves off a single adb command from managed_install.py (~150ms)

BUG=520082
Committed: https://crrev.com/93135fd37e66a5783580e12850f143675123c019
Cr-Commit-Position: refs/heads/master@{#345145}

[agrieve, 2015-08-20 02:12:17]

agrieve@chromium.org changed reviewers:
+ jbudorick@chromium.org

[jbudorick, 2015-08-23 02:21:25]

lgtm

[agrieve, 2015-08-24 18:48:52]

The CQ bit was checked by agrieve@chromium.org

[agrieve, 2015-08-24 18:48:53]

The patchset sent to the CQ was uploaded after l-g-t-m from
jbudorick@chromium.org
Link to the patchset: https://codereview.chromium.org/1304713003/#ps20001
(title: "rebase")

[commit-bot: I haz the power, 2015-08-24 18:49:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1304713003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1304713003/20001

[commit-bot: I haz the power, 2015-08-24 19:27:50]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-08-24 19:28:45]

Patchset 2 (id:??) landed as
https://crrev.com/93135fd37e66a5783580e12850f143675123c019
Cr-Commit-Position: refs/heads/master@{#345145}