Reduce header dependencies.

Reduce header dependencies.

This removes all Common headers from Reactor headers, making it
easier to use Reactor outside of SwiftShader.

Bug swiftshader:10

Change-Id: I35c5d138e781d4e911d206954d38e392714d4659
Reviewed-on: https://swiftshader-review.googlesource.com/7330
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>