Fix errors in MB when run for the first time on a new builder.

Fix errors in MB when run for the first time on a new builder.

The first time we run a build on a GN builder, it's possible
that MB will run and the build directory (//out/Release) may not
exist yet; in that case, we need to create the directory before
trying to write the list of runtime_deps that GN will need to read.

R=maruel@chromium.org
BUG=480053
Committed: https://crrev.com/c3441d156f8beb8cdd76880988132c197ddef2e1
Cr-Commit-Position: refs/heads/master@{#335793}

[Dirk Pranke, 2015-06-23 19:53:55]

Patchset #1 (id:1) has been deleted

[Dirk Pranke, 2015-06-23 19:54:37]

https://codereview.chromium.org/1204793002/diff/20001/tools/mb/mb_unittest.py
File tools/mb/mb_unittest.py (right):

https://codereview.chromium.org/1204793002/diff/20001/tools/mb/mb_unittest.py...
tools/mb/mb_unittest.py:19: self.cmds = []
the stuff related to self.cmds and TempFile() are fixes for unrelated lint
errors.

[M-A Ruel, 2015-06-23 20:42:39]

lgtm

[Dirk Pranke, 2015-06-23 20:43:32]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-06-23 20:44:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1204793002/20001

[commit-bot: I haz the power, 2015-06-23 20:52:19]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-23 20:52:20]

Exceeded global retry quota

[Dirk Pranke, 2015-06-23 21:11:56]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-06-23 21:14:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1204793002/20001

[commit-bot: I haz the power, 2015-06-23 23:01:47]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-06-23 23:02:59]

Patchset 1 (id:??) landed as
https://crrev.com/c3441d156f8beb8cdd76880988132c197ddef2e1
Cr-Commit-Position: refs/heads/master@{#335793}