Fix luci-go isolate support on OSX.

Fix luci-go isolate support on OSX.

The --extra-variable FOO BAR format is not supported by the Go implementation.
Switch to --extra-variable FOO=BAR. Using the second form is only a problem when
the value has spaces and only on Windows and this case is OSX only.

TBR=vadimsh@chromium.org
BUG=

Committed: https://crrev.com/ed6da5c8c4695faeb54aa354f48616cfd8207f71
Cr-Commit-Position: refs/heads/master@{#331218}

[M-A Ruel, 2015-05-22 23:57:04]

The CQ bit was checked by maruel@chromium.org

[M-A Ruel, 2015-05-22 23:57:48]

The CQ bit was unchecked by maruel@chromium.org

[commit-bot: I haz the power, 2015-05-22 23:57:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1152243003/1

[tandrii(chromium), 2015-05-23 00:00:37]

tandrii@chromium.org changed reviewers:
+ tandrii@chromium.org

[tandrii(chromium), 2015-05-23 00:00:38]

lgtm

[M-A Ruel, 2015-05-23 00:02:00]

The CQ bit was checked by maruel@chromium.org

[commit-bot: I haz the power, 2015-05-23 00:03:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1152243003/20001

[commit-bot: I haz the power, 2015-05-23 00:03:16]

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

[commit-bot: I haz the power, 2015-05-23 00:03:17]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[Vadim Sh., 2015-05-23 00:04:27]

lgtm, I hope python is fine with that

[M-A Ruel, 2015-05-23 00:04:34]

The CQ bit was checked by maruel@chromium.org

[commit-bot: I haz the power, 2015-05-23 00:05:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1152243003/20001

[commit-bot: I haz the power, 2015-05-23 00:13:19]

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

[commit-bot: I haz the power, 2015-05-23 00:13:20]

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

[M-A Ruel, 2015-05-23 00:22:23]

On 2015/05/23 00:13:20, I haz the power - commit-bot wrote:
> Try jobs failed on following builders:
>   chromium_presubmit on tryserver.chromium.linux (JOB_FAILED,
>
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

TBR'ing due to I being an OWNER.

[M-A Ruel, 2015-05-23 00:22:27]

The CQ bit was checked by maruel@chromium.org

[commit-bot: I haz the power, 2015-05-23 00:23:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1152243003/20001

[commit-bot: I haz the power, 2015-05-23 01:38:08]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-05-23 01:38:49]

Patchset 2 (id:??) landed as
https://crrev.com/ed6da5c8c4695faeb54aa354f48616cfd8207f71
Cr-Commit-Position: refs/heads/master@{#331218}