Refactor libGLESv2.gypi to allow libANGLE to be built on all platforms.

Required pulling angle_enable_d3d{9,11} into angle.gypi and definition
of angle_libangle_win_sources. Pull in those sources in BUILD.gn.

Tested with local GYP build on Windows. Did not test GN build yet.

BUG=angleproject:773

Change-Id: Ia3703fe4fa2bc8a1682c1fa1667ca36268b36cc7
Reviewed-on: https://chromium-review.googlesource.com/221785
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Kenneth Russell <kbr@chromium.org>
Use GL_APIENTRY and EGLAPIENTRY instead of __stdcall directly.

BUG=angleproject:773

Change-Id: Iab098f787b966bf3999c22687af113cc9909593b
Reviewed-on: https://chromium-review.googlesource.com/221796
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Kenneth Russell <kbr@chromium.org>