Add glTexStorage3DMultisampleANGLE entry point

This adds the entry point but does not implement it yet.

The entry point and enums are also added to the gl2ext_angle.h
header file.

BUG=angleproject:2775
TEST=angle_end2end_tests

Change-Id: I24c231c52e7cbb13637880b21044e655935b51e8
Reviewed-on: https://chromium-review.googlesource.com/1188575
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
diff --git a/src/libANGLE/Context.cpp b/src/libANGLE/Context.cpp
index 4cb2d41..48ae144 100644
--- a/src/libANGLE/Context.cpp
+++ b/src/libANGLE/Context.cpp
@@ -4994,6 +4994,17 @@
                                                ConvertToBool(fixedsamplelocations)));
 }
 
+void Context::texStorage3DMultisample(TextureType target,
+                                      GLsizei samples,
+                                      GLenum internalformat,
+                                      GLsizei width,
+                                      GLsizei height,
+                                      GLsizei depth,
+                                      GLboolean fixedsamplelocations)
+{
+    UNIMPLEMENTED();
+}
+
 void Context::getMultisamplefv(GLenum pname, GLuint index, GLfloat *val)
 {
     // According to spec 3.1 Table 20.49: Framebuffer Dependent Values,