Implementation of EGL_ANGLE_explicit_context. Includes new libGLESv2 entry
points and exports, libANGLE entry points, extension declarations for
eglGetProcAddress, and unit tests. Autogeneration scripts have been
modified to produce entry points, exports, eglGetProcAddress function
table, extension function pointers, and function declarations.

Bug:angleproject:1395
Change-Id: I1b79c6069bbed05beb4700a32139a64ddc465c4c
Reviewed-on: https://chromium-review.googlesource.com/1039865Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>