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

Review URL: https://codereview.chromium.org/1090333002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#27926}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh