Reset ES3 pixel pack parameters and PIXEL_PACK_BUFFER binding in DrawingBuffer before ReadPixels() and recover them later.

BUG=740603
TEST=new conformance test
R=kbr@chromium.org,piman@chromium.org

Change-Id: I3ea54c6cc34f34e249f7c8b9f792d93c5e1958f4
Reviewed-on: https://chromium-review.googlesource.com/570840
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486518}
bool WebGL2RenderingContextBase::ValidateCapability(const char* function_name,
                                                    GLenum cap) {
  switch (cap) {
    case GL_RASTERIZER_DISCARD:
      return true;
    default:
      return WebGLRenderingContextBase::ValidateCapability(function_name, cap);
  }
}

bool WebGL2RenderingContextBase::ValidateCapability(const char* function_name,
                                                    GLenum cap) {
  switch (cap) {
    case GL_RASTERIZER_DISCARD:
      return true;
    default:
      return WebGLRenderingContextBase::ValidateCapability(function_name, cap);
  }
}
