Add swarming support for MB (and the GN bots) in the chromium recipes.

Add swarming support for MB (and the GN bots) in the chromium recipes.

This patch adds support for swarming to bots that use the chromium
recipes (chromium.py, chromium_trybot.py) and the MB build wrapper
(so that we can use GN).

It does this by adding a hook to the run_mb() call that takes a
list of the gtest_test names (which need to match their ninja
build target names) and passes that to `mb gen --swarming-targets`
via a temp file.

MB then re-maps the ninja target names to the GN labels, and
writes a file containing the labels into the build directory,
and passes that to GN (or, it will ignore this in a GYP build).

Based on Dirk Pranke's CL https://codereview.chromium.org/1177593002

BUG=480053
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=295711

[Paweł Hajdan Jr., 2015-06-16 13:51:10]

phajdan.jr@chromium.org changed reviewers:
+ dpranke@chromium.org

[Paweł Hajdan Jr., 2015-06-16 13:51:11]

Patchset 1 should be same as your CL, and Patchset 2 contains my edits.

What do you think?

[Dirk Pranke, 2015-06-16 15:54:01]

lgtm. 

I'd prefer it if we could rename compile_specific_targets() and compile() to
something like 'build_specific_targets()' to slightly reduce the confusion that
the compile step is doing more than just compiling, but we can take care of that
in a follow-up CL if need be.

[Dirk Pranke, 2015-06-16 22:23:06]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-06-16 22:23:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1187773004/20001

[commit-bot: I haz the power, 2015-06-16 22:26:41]

Committed patchset #2 (id:20001) as
http://src.chromium.org/viewvc/chrome?view=rev&revision=295711