Implement GL_ANGLE_texture_multisample API part

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>
diff --git a/scripts/generate_entry_points.py b/scripts/generate_entry_points.py
index ae4ed83..3264344 100755
--- a/scripts/generate_entry_points.py
+++ b/scripts/generate_entry_points.py
@@ -53,6 +53,7 @@
     "GL_ANGLE_framebuffer_blit",
     "GL_ANGLE_framebuffer_multisample",
     "GL_ANGLE_instanced_arrays",
+    "GL_ANGLE_texture_multisample",
     "GL_ANGLE_translated_shader_source",
     "GL_EXT_blend_func_extended",
     "GL_EXT_debug_marker",