Merge MB from trunk back to M45.

Merge MB from trunk back to M45.

In order to be able to build the official linux x64 builds with GN
on trunk and GYP on the branches, we need to have the latest version
of MB on the branches for consistency (so that we can flip configs
without having to worry about not having all of the needful bug fixes
on the branch).

R=kerz@chromium.org
BUG=530733
Committed: https://chromium.googlesource.com/chromium/src/+/4d38e0ec3f03330b567fb572631ea99a30bdfaa0

[Dirk Pranke, 2015-09-26 00:43:55]

This is a straight copy of src/tools/mb@{#350953} 

(as per https://codereview.chromium.org/1366093004/)

the same comment applies; I can probably make this a much smaller patch if need
be, but I'd just as soon not have to worry about it.

[Dirk Pranke, 2015-09-28 18:26:58]

Committed patchset #1 (id:1) manually as
4d38e0ec3f03330b567fb572631ea99a30bdfaa0 (tree was closed).