Also add a build argument to build it as a shared library.

BUG=angleproject:2343

Change-Id: Ia590632b94d71b10d3ee64974fa4f5b4f00c3a70
Reviewed-on: https://chromium-review.googlesource.com/954403Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>