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/validationES31.cpp b/src/libANGLE/validationES31.cpp
index ee87f2a..9dc1230 100644
--- a/src/libANGLE/validationES31.cpp
+++ b/src/libANGLE/validationES31.cpp
@@ -2012,4 +2012,18 @@
     return true;
 }
 
+// GL_ANGLE_texture_multisample_array
+bool ValidateTexStorage3DMultisampleANGLE(Context *context,
+                                          TextureType target,
+                                          GLsizei samples,
+                                          GLint sizedinternalformat,
+                                          GLsizei width,
+                                          GLsizei height,
+                                          GLsizei depth,
+                                          GLboolean fixedsamplelocations)
+{
+    UNIMPLEMENTED();
+    return false;
+}
+
 }  // namespace gl