[NaCl] Define GetPageSize symbol for Gyp build
[NaCl] Define GetPageSize symbol for Gyp build

This is a follow-up to https://crrev.com/1391753003 which defined the
symbol for GN builds. The missing symbol did not cause any problems for
the Gyp build at the time, but once the code path starts being used in
future CLs, the Gyp build will require the symbol to de defined as well.

Review URL: https://codereview.chromium.org/1398293003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#353604}

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