[CSS Grid Layout] Fix grid shorthands CSSOM serialization

[CSS Grid Layout] Fix grid shorthands CSSOM serialization

Grid shorthands use slash instead of whitespace as separator. Modified
StylePropertySerializer::getShorthandValue() to allow specify a
separator different than whitespace.

TEST=fast/css-grid-layout/grid-shorthands-style-format.html

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=195148

[Manuel Rego, 2015-05-08 07:36:58]

rego@igalia.com changed reviewers:
+ dsinclair@chromium.org, jchaffraix@chromium.org, leviw@chromium.org

[dsinclair, 2015-05-08 13:18:13]

The CQ bit was checked by dsinclair@chromium.org

[dsinclair, 2015-05-08 13:18:13]

lgtm

[commit-bot: I haz the power, 2015-05-08 13:18:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1132753002/1

[commit-bot: I haz the power, 2015-05-08 16:14:14]

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

[commit-bot: I haz the power, 2015-05-08 16:14:16]

Try jobs failed on following builders:
  win_blink_rel on tryserver.blink (JOB_FAILED,
http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/62180)

[Manuel Rego, 2015-05-08 20:15:22]

The CQ bit was checked by rego@igalia.com

[Manuel Rego, 2015-05-08 20:15:23]

The patchset sent to the CQ was uploaded after l-g-t-m from
dsinclair@chromium.org
Link to the patchset: https://codereview.chromium.org/1132753002/#ps20001
(title: "Update test expectations for webexposed/nonstable-css-properties.html")

[commit-bot: I haz the power, 2015-05-08 20:15:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1132753002/20001

[commit-bot: I haz the power, 2015-05-09 10:30:12]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=195148