Validate GL backend after dispatch table initialization

This commit adds a check to ensure that the GL backend's functions
have been properly initialized. This may happen with third party
build toolchains, like vcpkg, which reimplement parts of the
existing build system.

Bug: angleproject:8195
Change-Id: Iaca2200a563c5049d90acad57785088c94b4e580
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4614645
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>

src/libANGLE/renderer/gl/FunctionsGL.cpp

diff --git a/src/libANGLE/renderer/gl/FunctionsGL.cpp b/src/libANGLE/renderer/gl/FunctionsGL.cpp
index 7f80977..737e8d0 100644
--- a/src/libANGLE/renderer/gl/FunctionsGL.cpp
+++ b/src/libANGLE/renderer/gl/FunctionsGL.cpp
@@ -156,6 +156,9 @@
 #endif  // defined(ANGLE_ENABLE_GL_NULL)
             {
                 initProcsDesktopGL(version, extensionSet);
+                // Test that ANGLE_ENABLE_GL_DESKTOP_BACKEND has been enabled
+                // See http://anglebug.com/8195
+                ASSERT(getString != nullptr && getError != nullptr);
             }
             break;
         }