Use cross-platform APIs and constants, and conditionally compile the
Windows-specific DllMain.

BUG=angleproject:783

Change-Id: I8fd2708ab0925cb3207010eb0e759cfc055183ab
Reviewed-on: https://chromium-review.googlesource.com/222955Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Kenneth Russell <kbr@chromium.org>