Support GL_ANGLE_texture_multisample extension.
This patch adds enums of multisampled texture and texStorage2DMultisampleANGLE
API.

TEST=angle_end2end_tests.exe --gtest_filter=TextureMultisampleTest*
TEST=angle_end2end_tests.exe --gtest_filter=NegativeTextureMultisampleTest.Negtive*

BUG=angleproject:2275

Change-Id: I2cab997edc33aa2d0be6082381545335423f64e0
Reviewed-on: https://chromium-review.googlesource.com/c/804613
Commit-Queue: Yizhou Jiang <yizhou.jiang@intel.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>