Adding missing V8_EXPORT flag in SpaceStatistics class in v8.h

Adding missing V8_EXPORT flag in SpaceStatistics class in v8.h

This class was added in crrev.com/1058253003 and missed V8_EXPORT in
definition.

BUG=466141, 476013
LOG=Y

Committed: https://crrev.com/4a597f527a657846210edcc69e2b20ff1ba02afd
Cr-Commit-Position: refs/heads/master@{#27926}

[ssid, 2015-04-17 18:37:22]

ssid@chromium.org changed reviewers:
+ primiano@chromium.org

[ssid, 2015-04-17 18:38:02]

ssid@chromium.org changed reviewers:
+ hpayer@chromium.org

[Hannes Payer (out of office), 2015-04-17 18:40:12]

LGTM

[Primiano Tucci (use gerrit), 2015-04-17 18:45:56]

The CQ bit was checked by primiano@chromium.org

[commit-bot: I haz the power, 2015-04-17 18:46:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1090333002/1

[commit-bot: I haz the power, 2015-04-17 19:33:25]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-17 19:33:38]

Patchset 1 (id:??) landed as
https://crrev.com/4a597f527a657846210edcc69e2b20ff1ba02afd
Cr-Commit-Position: refs/heads/master@{#27926}