This patch implements FRAMEBUFFER_DEFAULT_LAYERS_EXT as a legal framebuffer
parameter required in OpenGL ES 3.1 extension EXT_geometry_shader on OpenGL
back-ends.

The query on FRAMEBUFFER_DEFAULT_LAYERS_EXT specifies the layer count used
when the framebuffer has no attachments.

BUG=angleproject:1941
TEST=dEQP-GLES31.functional.geometry_shading.query.framebuffer_default_layers

Change-Id: I888465dfe23da53541ec2fedb8616027df532466
Reviewed-on: https://chromium-review.googlesource.com/1063560Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>