BUG=angleproject:1183

This is a reland of part of commit 56627322.
The original commit caused warnings on 64-bit MSVC linker. This version
removes declspec(dllexport) from exported ANGLE APIs (for which we need
a new ANGLE_PLATFORM_EXPORT macro). It also makes those APIs stdcall on
Windows builds.

Change-Id: Iee2ab7e43ef0b98924f5759b5e1b333307052235
Reviewed-on: https://chromium-review.googlesource.com/304501Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>