Have HWUI validate the WebViewFunctors at registration

Also tweak how webview's plat_support creates the functor
sruct to be thread-safe, avoids any potential race conditions
even though WebView itself is stated to be thread-hostile in
general. It's too easy to have this just be defined-safe instead.

Bug: 186814981
Test: build & boot, no crashes in real-webview apps
Change-Id: I06f02a279e248fee375ce133c5ce9a2250665ad9
diff --git a/libs/hwui/tests/unit/main.cpp b/libs/hwui/tests/unit/main.cpp
index 402cb58..10c874e 100644
--- a/libs/hwui/tests/unit/main.cpp
+++ b/libs/hwui/tests/unit/main.cpp
@@ -62,8 +62,10 @@
         gSigChain.insert(pair<int, struct sigaction>(sig, old_sa));
     }
 
-    // Replace the default GLES driver
+    // Avoid talking to SF
     Properties::isolatedProcess = true;
+    // Default to GLES (Vulkan-aware tests will override this)
+    Properties::overrideRenderPipelineType(RenderPipelineType::SkiaGL);
 
     // Run the tests
     testing::InitGoogleTest(&argc, argv);