A change in Chromium's build tools caused symbol paths to become
relative. Since copying dlls no longer links back to the build
directory we need to copy the PDBs as well to get debuggging symbols.

Bug: angleproject:3001
Change-Id: I54d9a1d57d3c69d81d68913993653bab61fa4e65
Reviewed-on: https://chromium-review.googlesource.com/c/1358645Reviewed-by: Jamie Madill <jmadill@chromium.org>