Implement TexStorage3DMultisample on the GL backend

BUG=angleproject:2775
TEST=angle_end2end_tests

Change-Id: Ic980d86cd787bcf29f622e68b0c38b0eb6ca5688
Reviewed-on: https://chromium-review.googlesource.com/1190182
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 d428b84..29b5e46 100644
--- a/src/libANGLE/Context.cpp
+++ b/src/libANGLE/Context.cpp
@@ -5010,7 +5010,10 @@
                                       GLsizei depth,
                                       GLboolean fixedsamplelocations)
 {
-    UNIMPLEMENTED();
+    Extents size(width, height, depth);
+    Texture *texture = getTargetTexture(target);
+    handleError(texture->setStorageMultisample(this, target, samples, internalformat, size,
+                                               ConvertToBool(fixedsamplelocations)));
 }
 
 void Context::getMultisamplefv(GLenum pname, GLuint index, GLfloat *val)