Fix various windows-isms in the MB gen code paths.

Fix various windows-isms in the MB gen code paths.

We needed to handle windows-style path separators
properly, and handle the fact that binaries have ".exe"
in their names.

R=maruel@chromium.org
BUG=480053
Committed: https://crrev.com/34bd39d5ca100f9417423cedc6d0228af73acd35
Cr-Commit-Position: refs/heads/master@{#335859}

[M-A Ruel, 2015-06-23 21:10:45]

lgtm

[Dirk Pranke, 2015-06-23 21:12:47]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-06-23 21:14:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1204863002/1

[commit-bot: I haz the power, 2015-06-23 21:29:00]

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

[commit-bot: I haz the power, 2015-06-23 21:29:01]

Try jobs failed on following builders:
  linux_chromium_gn_rel on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Dirk Pranke, 2015-06-24 01:29:53]

The CQ bit was checked by dpranke@chromium.org

[Dirk Pranke, 2015-06-24 01:29:54]

The patchset sent to the CQ was uploaded after l-g-t-m from maruel@chromium.org
Link to the patchset: https://codereview.chromium.org/1204863002/#ps20001
(title: "s/\\/os.sep/")

[commit-bot: I haz the power, 2015-06-24 01:31:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1204863002/20001

[commit-bot: I haz the power, 2015-06-24 02:36:59]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-24 02:38:07]

Patchset 2 (id:??) landed as
https://crrev.com/34bd39d5ca100f9417423cedc6d0228af73acd35
Cr-Commit-Position: refs/heads/master@{#335859}