ES31: Add PROGRAM_SEPARABLE to ProgramParameter and GetProgram

BUG=angleproject:1939
TEST=angle_end2end_tests

Change-Id: I97ad11360f7c015947a2c0cc7d4a47f994726834
Reviewed-on: https://chromium-review.googlesource.com/454264
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/libANGLE/renderer/null/ProgramNULL.h b/src/libANGLE/renderer/null/ProgramNULL.h
index 502862f..88c0598 100644
--- a/src/libANGLE/renderer/null/ProgramNULL.h
+++ b/src/libANGLE/renderer/null/ProgramNULL.h
@@ -26,6 +26,7 @@
                     gl::BinaryInputStream *stream) override;
     gl::Error save(gl::BinaryOutputStream *stream) override;
     void setBinaryRetrievableHint(bool retrievable) override;
+    void setSeparable(bool separable) override;
 
     LinkResult link(ContextImpl *contextImpl,
                     const gl::VaryingPacking &packing,