Define symbol classes in a separate file The plan is to use symbols more also outside the symbol table, so it makes sense to define the symbol classes in a separate header file. BUG=angleproject:2267 TEST=angle_unittests Change-Id: I94167415ef43ba9bd9126ca32d9c498e1437f3f8 Reviewed-on: https://chromium-review.googlesource.com/822414 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>