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/822414Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>