Fix stray ANGLE_EXPORT.

BUG=angle:733

Change-Id: Ia4a84bea483ac11a2326d2402649affe10d661c9
Reviewed-on: https://chromium-review.googlesource.com/232942
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>