fuchsia: Add vulkan loader service to sandbox policy

Since 57a19db ("[fuchsia] Whitelist required Fuchsia services in
component manifest.") vulkan fails to initialize. Fix it by adding
the vulkan loader service to the policy.

Bug: 861853
Test: content_shell --ozone-platform=scenic --enable-vulkan --disable-gpu

Change-Id: I38c11f32e0a0dd8b914184a16198c023877a0572
Reviewed-on: https://chromium-review.googlesource.com/1228488
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592126}

build/config/fuchsia/testing_sandbox_policy

@@ -10,6 +10,7 @@
       "fuchsia.process.Launcher",
       "fuchsia.ui.policy.Presenter",
       "fuchsia.ui.scenic.Scenic",
-      "fuchsia.ui.viewsv1.ViewManager"
+      "fuchsia.ui.viewsv1.ViewManager",
+      "fuchsia.vulkan.loader.Loader"
   ]
 }

webrunner/app/sandbox_policy

@@ -8,6 +8,7 @@
       "fuchsia.netstack.Netstack",
       "fuchsia.process.Launcher",
       "fuchsia.ui.scenic.Scenic",
-      "fuchsia.ui.viewsv1.ViewManager"
+      "fuchsia.ui.viewsv1.ViewManager",
+      "fuchsia.vulkan.loader.Loader"
   ]
 }